To complete the image, we will now add an adjustment layer so that we can make the foreground layer match the new brighter sky we added. Create New Fill or Adjustment Layer and from drop-down list in the layers palette, choose "Levels".

I also used the "blurr" tool to soften the hard edges, particularly around the coat area. A change of background transformed this image quite dramatically. Save to save as a .psd or flatten the image to save as a other file type.

Other resources you might wish to consult:
Guru's Network: http://www.gurusnetwork.com/tutorials/photoshop/replacebg.html