Don't try to change the way keys are mapped in edit mode unless you have a good reason.  It's complicated and easy to mess up.
If you do make a mistake, you can go back to the default edit mode keymapping just by removing the "User Data Folder/Save/EditMode_Keymaps.ini" file.

Edit Mode loads key mapping settings from "User Data Folder/Save/EditMode_Keymaps.ini".  "User Data Folder" is a special name that means the folder where all your user data for stepmania is stored, the name depends on what OS you're on.  See Docs/Userdocs/sm5_beginner.txt and search for "User Data Folder" to learn where it is on your OS.

Entries in EditMode_Keymaps.ini set which what keys trigger an action.  Any function that doesn't have an entry in EditMode_Keymaps.ini will use the default mapping.  Be careful when choosing a key for a function to make sure that same key isn't already mapped to something else either in the default mapping or in the mapping you are creating.
If you hold F3, you can use the Debug Menu to reload the screen while in Edit Mode, which will reload the key mapping, so you can adjust the mapping while stepmania is running.

EditMode_Keymaps.ini is broken up into four sections, one for each major mode that Edit Mode can be in.  The "Edit" section is for when in normal editing mode.  The "Play" section is for when Edit Mode is playing back a song.  The "Record" section is for record mode, and the "RecordPaused" section is for when Record mode is paused.

Each line in a section is the name of the function being set, followed by "=", then the name of the key for that function.  You can list two keys by seperating them with a ":".  Examples:
SNAP_NEXT=Key_s
SNAP_PREV=Key_d:Key_v

Note that you cannot change which keys need to be held down for certain things, like holding shift for rolls.

Next will be a list of function names (headed by "Function Names"), then a list of key names (headed by "Key Names"), then the default key mapping (headed by "Default Mapping") as an example.

Function Names:
COLUMN_0
COLUMN_1
COLUMN_2
COLUMN_3
COLUMN_4
COLUMN_5
COLUMN_6
COLUMN_7
COLUMN_8
COLUMN_9
RIGHT_SIDE
LAY_ROLL
LAY_TAP_ATTACK
REMOVE_NOTE
CYCLE_TAP_LEFT
CYCLE_TAP_RIGHT
CYCLE_SEGMENT_LEFT
CYCLE_SEGMENT_RIGHT
SCROLL_UP_LINE
SCROLL_UP_PAGE
SCROLL_UP_TS
SCROLL_DOWN_LINE
SCROLL_DOWN_PAGE
SCROLL_DOWN_TS
SCROLL_NEXT_MEASURE
SCROLL_PREV_MEASURE
SCROLL_HOME
SCROLL_END
SCROLL_NEXT
SCROLL_PREV
SEGMENT_NEXT
SEGMENT_PREV
SCROLL_SELECT
LAY_SELECT
SCROLL_SPEED_UP
SCROLL_SPEED_DOWN
SNAP_NEXT
SNAP_PREV
OPEN_EDIT_MENU
OPEN_TIMING_MENU
OPEN_ALTER_MENU
OPEN_AREA_MENU
OPEN_BGCHANGE_LAYER1_MENU
OPEN_BGCHANGE_LAYER2_MENU
OPEN_COURSE_MENU
OPEN_COURSE_ATTACK_MENU
OPEN_STEP_ATTACK_MENU
ADD_STEP_MODS
OPEN_INPUT_HELP
BAKE_RANDOM_FROM_SONG_GROUP
BAKE_RANDOM_FROM_SONG_GROUP_AND_GENRE
PLAY_FROM_START
PLAY_FROM_CURSOR
PLAY_SELECTION
RECORD_FROM_CURSOR
RECORD_SELECTION
RETURN_TO_EDIT
INSERT
DELETE
INSERT_SHIFT_PAUSES
DELETE_SHIFT_PAUSES
OPEN_NEXT_STEPS
OPEN_PREV_STEPS
PLAY_SAMPLE_MUSIC
BPM_UP
BPM_DOWN
STOP_UP
STOP_DOWN
DELAY_UP
DELAY_DOWN
OFFSET_UP
OFFSET_DOWN
SAMPLE_START_UP
SAMPLE_START_DOWN
SAMPLE_LENGTH_UP
SAMPLE_LENGTH_DOWN
ADJUST_FINE
SAVE
UNDO
ADD_COURSE_MODS
SWITCH_PLAYERS
SWITCH_TIMINGS


Key Names:
(If you're really having trouble finding the name for a key, map it to something in the normal controller mapping screen and check Keymaps.ini to see what its name is.)
Most keys can be set with something like this: "Key_a", where "a" is the character typed by the key.  This includes keys like ".", ",", and so on.  For other keys, there are special names that can be used, which are listed below.  "Key_." and "Key_period" both mean the same thing.
Some key names have spaces in them.

Key_period
Key_comma
Key_space
Key_delete
Key_backspace
Key_tab
Key_enter
Key_pause
Key_escape
Key_F1
Key_F2
(Don't map F3, it's used for the debug menu, so probably won't work.)
Key_F4
Key_F5
Key_F6
Key_F7
Key_F8
Key_F9
Key_F10
Key_F11
Key_F12
Key_F13
Key_F14
Key_F15
Key_F16
Key_left ctrl
Key_right ctrl
Key_left shift
Key_right shift
Key_left alt
Key_right alt
Key_left meta
Key_right meta
Key_left super
Key_right super
Key_menu
Key_function
Key_num lock
Key_scroll lock
Key_caps lock
Key_prtsc
Key_up
Key_down
Key_left
Key_right
Key_insert
Key_home
Key_end
Key_pgup
Key_pgdn
Key_KP 0
Key_KP 1
Key_KP 2
Key_KP 3
Key_KP 4
Key_KP 5
Key_KP 6
Key_KP 7
Key_KP 8
Key_KP 9
Key_KP /
Key_KP *
Key_KP -
Key_KP +
Key_KP .
Key_KP =
Key_KP enter


Default Mapping:
Comments are in this section to tell which things require a modifier key to be held.
Note that not all sections map all functions.  This is because those functions aren't useful in all modes.
(if you spot a key name that is in all caps in this section, I missed it when editing at 4 AM and it's supposed to be lower case)

[Edit]
SCROLL_UP_LINE=Key_up
SCROLL_DOWN_LINE=Key_down
SCROLL_UP_PAGE=Key_pgup:Key_;
SCROLL_DOWN_PAGE=Key_pgdn:Key_'
# Scrolling by timing segments actually requires holding Ctrl.
SCROLL_UP_TS=Key_pgup:Key_;
SCROLL_DOWN_TS=Key_pgdn:Key_'
SCROLL_HOME=Key_home
SCROLL_END=Key_end
SCROLL_NEXT=Key_period
SCROLL_PREV=Key_comma
# Hold Ctrl for SEGMENT_NEXT and SEGMENT_PREV.
SEGMENT_NEXT=Key_period
SEGMENT_PREV=Key_comma
SCROLL_SELECT=Key_left shift:Key_right shift
LAY_SELECT=Key_space
# Hold Ctrl for PLAY_FROM_START.
PLAY_FROM_START=Key_p
# Hold Shift for PLAY_FROM_CURSOR.
PLAY_FROM_CURSOR=Key_p
PLAY_SELECTION=Key_p
OPEN_TIMING_MENU=Key_F4
# OPEN_PREV_STEPS/OPEN_NEXT_STEPS aren't allowed in home mode. It breaks the "delay creation until first save" logic.
OPEN_PREV_STEPS=Key_F5
OPEN_NEXT_STEPS=Key_F6
BPM_DOWN=Key_F7
BPM_UP=Key_F8
STOP_DOWN=Key_F9
STOP_UP=Key_F10
# Hold Shift for DELAY_DOWN and DELAY_UP.
DELAY_DOWN=Key_F9
DELAY_UP=Key_F10
OFFSET_DOWN=Key_F11
OFFSET_UP=Key_F12
SAMPLE_START_UP=Key_]
SAMPLE_START_DOWN=Key_[
# Hold Shift for SAMPLE_LENGTH_UP and SAMPLE_LENGTH_DOWN.
SAMPLE_LENGTH_UP=Key_]
SAMPLE_LENGTH_DOWN=Key_[
PLAY_SAMPLE_MUSIC=Key_l
OPEN_BGCHANGE_LAYER1_MENU=Key_b
# Hold Shift for OPEN_BGCHANGE_LAYER2_MENU.
OPEN_BGCHANGE_LAYER2_MENU=Key_b
# Hold Ctrl for INSERT_SHIFT_PAUSES and DELETE_SHIFT_PAUSES.
INSERT_SHIFT_PAUSES=Key_insert
DELETE_SHIFT_PAUSES=Key_DEL
COLUMN_0=Key_1
COLUMN_1=Key_2
COLUMN_2=Key_3
COLUMN_3=Key_4
COLUMN_4=Key_5
COLUMN_5=Key_6
COLUMN_6=Key_7
COLUMN_7=Key_8
COLUMN_8=Key_9
COLUMN_9=Key_0
# Yeah, you're screwed when editing techno-double8, bm-double5, or bm-double7, or any other style with more than 10 columns.
RIGHT_SIDE=Key_left alt:Key_right alt
LAY_ROLL=Key_left shift:Key_right shift
CYCLE_TAP_LEFT=Key_n
CYCLE_TAP_RIGHT=Key_m
# Hold Ctrl for CYCLE_SEGMENT_LEFT and CYCLE_SEGMENT_RIGHT.
CYCLE_SEGMENT_LEFT=Key_n
CYCLE_SEGMENT_RIGHT=Key_m
# Hold Ctrl for SCROLL_SPEED_UP and SCROLL_SPEED_DOWN
SCROLL_SPEED_UP=Key_up
SCROLL_SPEED_DOWN=Key_down
SCROLL_SELECT=Key_left shift:Key_right shift
SNAP_NEXT=Key_left
SNAP_PREV=Key_right
OPEN_EDIT_MENU=Key_escape
OPEN_AREA_MENU=Key_enter
OPEN_ALTER_MENU=Key_a
OPEN_INPUT_HELP=Key_F1
# Hold Alt for BAKE_RANDOM_FROM_SONG_GROUP.
BAKE_RANDOM_FROM_SONG_GROUP=Key_b
# Hold Ctrl for BAKE_RANDOM_FROM_SONG_GROUP_AND_GENRE.
BAKE_RANDOM_FROM_SONG_GROUP_AND_GENRE=Key_b
# Hold Ctrl for RECORD_SELECTION.
RECORD_SELECTION=Key_r
INSERT=Key_insert:Key_\
DELETE=Key_delete
ADJUST_FINE=Key_right alt:Key_left alt
# Hold Ctrl or Cmd (OS X) for SAVE.
SAVE=Key_s
UNDO=Key_u
SWITCH_PLAYERS=Key_/
SWITCH_TIMINGS=Key_t

[Play]
RETURN_TO_EDIT=Key_enter:Key_escape

[Record]
LAY_ROLL=Key_left shift:Key_right shift
REMOVE_NOTE=Key_left alt:Key_right alt
RETURN_TO_EDIT=Key_escape:Key_enter

[RecordPaused]
PLAY_SELECTION=Key_p
# Hold Ctrl for RECORD_SELECTION.
RECORD_SELECTION=Key_r
RECORD_FROM_CURSOR=Key_r
RETURN_TO_EDIT=Key_escape
UNDO=Key_u
