Once inside the room you need to open the housing menu (Shift-H for me, although it's possible I made that binding myself a long time ago). On the menus it's under Social then Housing. Then you get another menu pop up, Indoor Furnishings is the top option and that's where you add, rotate, remove items.

Remodel Interior is the second option that will allow you to change the walls, floor and light fitting, but please be aware (if you weren't already) that once placed and confirm, you can't ever get the items back, you can only overwrite them. It's okay to put a wall/floor/light in to see how it looks, just don't press confirm, at that point it uses it.