I know this is a very stupid question that I shouldn’t be asking but- I have no idea how do I do that, so how do I change a buttons background with a texture?
You need to modify the Button’s Target Graphic. You can do it through code, like in this link, or in the editor. Just add an Image on the same GameObject, and the Button will detect it automatically.
You can also use the Transition parameter to swap sprites or apply a color tint for different states (normal, disabled, hovering, clicked…) .