Just a heads up, since the controls are only included in the details.
You can open the level editor by pressing enter and save any changes to the level using v. Pressing enter again will bring you back to the game state.For greater detail, check the game’s detail page. For an example level and details on how to share your levels, check out user-level.fnl.
Pressing p will pause the game, m will mute the music, q will take a screenshot and r will reset the level (useful when testing your level designs).
If you have to quit the game, or get stuck on a level you can use the “load level” option to open any of the levels (including your own custom level).
Also, check out the options menu. You can toggle on and off mouse lock, which will bind the mouse to the screen, and toggle on and off mouse visibility. It might be easier to use the editor with the mouse visibility enabled.