Today I'd like to write a useful blog; so I hope what I'm going to write will be a good tip expecialy for newby and for the ones not so able with photography.
It would be a great pity not to change the sky and not to turn it n a great image... luckly often it's possible to ge it ;)
Open your image with Photoshop and then do all these things:
1- Duplicate the background layer (this is very important, 'cause in case of mistake you always have your first step to use).
2- Create a new layer and call it "sky"
3- with the wizard instrument select the sky of the background (use tolerance around 30, so that you don't select other parts of the pictures except the sky)
4- Fill the selected area on the layer called "sky" with a beautiful cyan (you can also try to get great effect with lilac, dark blue,...)
5- Turn the layer "sky" on the way "multiply"
6- Change its opacity between 10 and 50 (it has to be up to the background of course, change its position if necessary)
7- select the inver of the sky and change the contrast and the light of the rest part of the background. Usually you have to add ligh and its important to don't add light to the sky.