The wiki is a bit too technical and jargon-y for me to make sense of it, to if the answer is in there, it's not helpful to someone as dumb as me.

I just want to change an item from another mod so that it goes into the cape slot instead of the back slot. Is that something I can even do?

Depends on the mod and how they implement the Accessory combined with if the mod uses Accessories though the compat layer for trinkets or curios. All in all in most cases it requires adjusting the tag inwhich the item is part meaning you will need to put it in the accessories:back tag instead of the accessories:cape tag.

Next time please ask this question within the Discord linked in the github, modrinth or curseforge instead of making a issue about this.