In this tutorial I will show you how you can change the environment of a picture.

STEP 1.
After you decided on the picture you want to modify the background, and you decided what you want to keep and what to throw away, go to Filter and select Extract(Alt+Ctrl+X) then select Edge Highlighter Tool(B), mark the Smart Highlighting box from the Tool Options on your right and start to select the objects you want to keep. In this case the building.

STEP 2.
Press the Fill Tool(G) and click inside the selection. If it turns blue it means you have a close selection, if not then it means you have a gap somewhere in the selection. After filling the selection the press ok.

STEP 3.
Using History Brush Tool(Y) repair any edges that have not come out really great. Then you can select all picture, then press the Wand button (W), make sure the subtract from selection option is selected then press on the white part of the picture, thus living you just with the buiding selected. Copy it and put it in the new environment picture. But i did the oposite and brought the environment over the building picture.