Generally state what version of Vue your using helps, And thats what this forum is here for asking Q's. In the backroom (lookup at the header) there are plenty of tutorials I have found very helpful...

Okay so you want to put a different top on a (cube) building, most of the models have a (+) symbol in the right side viewer, this will expand to individual components of the object. click a highlight on a component and change its texture.

And play with some of the free models in the FreeStuff ....

hope it helps

R