Tutorial: Changing the backdrop
Easy step by step, Hardest part is finding the new background images!
       
 Changing the backdrop
 
Difficulty - **

We want to add a backdrop to this convertible.
Get a new background backdrop.. such as a field, or a sky.. (try searching on "Google image search" for "landscape" or "backdrop" etc. and then paste it over the top half of the picture.  
Adjust the opacity of the new layer (using the layer box in bottom right) to around 50% so you can see through to the background..  

Right, now delete the areas where you wouldn't see the background (hashed out on image).  
Almost complete now.. change the opacity back to 100. As you see, the area around the window looks strange. This is due to the fact that the window changes the colour of the background..  
So, use the lasso tool, and select the areas as shown.  

Then go to image>adjust>brightness/contrast and adjust the sliders until the image looks correct behind the window. and your done! In this case, I lowered the contrast and increased the brightness.