How can Petmancer be missing a critical feature like the ability to customize keyboard assignments? It can't, and now it isn't!
How to Find It
In the Settings screen, next to the Control Type you'll now see a Remap Keys button, which will take you to the Keyboard Controls screen shown above.
Click on the key assignments, or navigate to the key shown and press Enter, and you'll be able to press the key you want to reassign the action to.
The only actions you can't change are the arrow keys, Esc for the menu, and some standard UI navigation keys (Esc to exit, Enter or Space to activate buttons, Tab / Shift-Tab to move around, etc).
Give it a try and send feedback if you need any additional tweaks!