Go into:

Steam\steamapps\common\Rabbit and Steel\Animations

and change whichever one you want to use
(make a copy of the original or the entire folder itself and move it out onto your desktop or whatever!)

I just drew a black line over top of the skin to test it but you can replace it with whatever you want.

Just note that the sprites aren't squared off or anything so make sure it doesn't poke too far into the next section of animation

The squares are 500x500 in size but the entire file for the skin is 12500x500 so be careful you don't go too far into the next 500x500 square

I used PAINTnet but you can open it with whatever editing software you want, it's very easy to do :3

feel free to ask if you need help i will reply



* Apparently you can also use "UndertaleModTool" on github (by UnderMinersTeam) but I don't personally have experience with it



+++!!!
The developer has released the sprites on the discord just search ".clip"

https://discord.gg/mns <--- Invite link

You will need to use Clip Studio Paint as far as I'm aware

I don't know if you can convert the file types to other programs but feel free to try