As basic as it sounds.

Image source: https://rougelabatte.tumblr.com/post/659262271182307328/monika-icon-transparent-pls-credit-if

Instruction: Take the folder out of the zip file and put it in the assets\mod_overrides . If mod_overrides does not exist create it

Now you can download an updated version of the mod which was made by @ghoster-1