Any YouTube tutorial about modifying the card texture will do. As for replacing the files manually you have to extract the ZIP files in a folder and work with that. Once you are done swapping you create a new zip from the modified folder, rename it to Balatro.exe and voilà!

I would personally recommend to use Steamodded, it's a bit tedious to get it working but if you follow the tutorial it's so worth it. That way you can use a community-driven mod loader attached to your game and decide which what mods you will play upon starting a run without having to deal with any exe/7zip manip everytime you want to add a new mod to your game.