The key to a successful military campaign is flexibility. Therefore, most of your buildings are not permanent structures. They can be collapsed, with their components loaded onto transport vehicles, and moved to more strategic locations in short order. When a building is on transport vehicles, it can receive movement orders just like a regiment. Click on the Unpack icon [P] when you are ready to assemble your building. Move your mouse to the location where you want to assemble the building, click on a location on the landscape and the transports will move to that location and unpack.

Note: If the building turns red, you are trying to place it somewhere that it cannot go. It may be too close to another building, on a mountain, in the water and so on. If you want to move a building to a different location, just click on the Pack icon [P] and the building will return to the transport vehicles, ready to be moved to a new spot.



Source: Axis and Allies Official Manual