Hey! Right now there's no config file to edit, but I suspect there will be in the near future.
In the meantime, if you're not opposed to building the game yourself (which I know can be a bit daunting if you haven't before) you can edit this file:
https://github.com/ExOK/Celeste64/blob/main/Source/Controls.cs
Alternatively joy2key is always a fallback until there's a config file.