You have to download interior extensions from the workshop, a lot of them can be found by just typing "TARDIS" and looking at top rated. However, the Capaldi one is called like "Capaldi Extension" or something like that.

Once you've installed some new interiors, go to the "options" tab in the top right of the spawn menu(the menu that comes up when you hold q), look for TARDIS Rewrite. From there you can choose to pop-out the screen(The one that's inside the default TARDIS Rewrite), then just look for something named "interiors" and select one you want. Once selected, delete the TARDIS and spawn in a new one. Then it should be done.

This is all off memory so I hope it helps.

Thank you so much!