Tutorial: Convertible Conversion
Easy way to remove the roof in Photoshop, this is quite hard, but once the principles are learnt, you can easily practice.
       
 Convertible conversion
 
Difficulty - ****
  a First, we need a donor open top car in the same position as the car we are converting.

Here, we can fit the Astra Convertible onto the Toyota Corrolla.
 
Draw around the convertible area like so, with the lasso tool. Copying it, then pasting it onto the car to be modified.  

Ok.. once you've done that, hit Ctrl+T and transform, skew, resize etc etc until the roof fits the rest of the bodywork. Then use the eraser tool (E) to slowly blend the roof into the bodywork.  

Next up, we need to successfully colour match the bodywork to the car, to do this, select the appropriate areas with the lasso tool, and then go to image>adjust>variations and adjust the levels until a perfect match is found!

Ok, once the roof looks part of the car, we can move on. Flatten all layers (Ctrl+Shift+E) so it looks like the piccy on the left! The thing that looks crap about this image is the messed up background and the sections in the windows. This can be cured by creating a new backdrop.
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.