1. Copy a userdata.lua file from one of the directories (you probably want SixPlayerKeyboardConfig, read CONTROLS.md in that directory for all the controls for each player).
2. Go to your Pico Park game install location e.g. C:\Program Files (x86)\steamapps\steamapps\common\PICO_PARK\save
3. Overwrite userdata.lua with the downloaded file (bear in mind this will wipe any hi-scores you have. Replace only the keyboardConfig= section with the downloaded one if you have any scores you want to keep).

Players 2+:

{
    22, // ACTION?
    88, // UNUSED
    22, // JUMP
    22, // ACTION?
    18, // DOWN
    0,  // LEFT
    3,  // RIGHT
    22, // ACTION?
    88, // UNUSED
    88, // UNUSED
},

I didn't realise there was an in-game keyboard control editor so I spent time decoding what the IDs bound. In case it is of any use this is as far as I got: