How do you change your wallpaper on Habbo?

If you mean your Rooms' walls then you first need to purchase coins. After purchasing coins or "credits." You can open the Catelogue and go to "Spaces". There you can purchase wall paper and flooring and also a window background. Once you purchase these, open your Inventory and go to "Floor". It'll look like a rolled up piece of carpet. Besure you are in the room you want to use the flooring with.