How to change background
This example will teach you to change background of any object, it is very easy by my this example so follow this step by step tutorial.
Picture: Take a picture to change background.
Selection: Make selection by the Magic wand tool (W key) as I have done here.
Make copy of the another picture to make background picture.
Paste: Go to Edit menu > Paste into (Shift + Ctrl + V).
Levels: Go to Image menu > Adjustment > Levels.
This is your final result.