1. Select the sky you want to replace
2. Invert the selection (Select > Inverse
3. Put the selection on his own Layer Ctrl or Cmd J
4. Drag the new sky over the image and make it fit using Ctrl or Cmd T
5. Drag the new sky under the layer of step 3
In this case I added some reflections of the clouds into the sky. I made the top of the sky a little darker and lightened up the photo with a Levels adjustment layer.
And voila: this is the end result: I think replacing the sky made the picture a whole lot more interesting.