Click your rental box and open the web page presented, either in the in-world browser or any external browser
Click [Skybox] on the left hand side of the page
At the bottom of the main area of the page are 3 links, one for selecting a pre-supplied texture for the wall, one for selecting a pre-supplied texture for the floor, or one for supplying your own texture. Pick as appropriate.
Pre-supplied textures are shown in the webpage as a grid, pick the appropriate “Select” button and the texture will be applied
Custom (user supplied) textures, once enabled, can simply be dragged while holding CONTROL on your keyboard, and dropped onto the appropriate skybox piece to retexture it. Make sure your texture is full permissions (or at least copy/transfer!)