Ok, so you shot a great photo on a clear day but you wished the sky was not as boring. There is nothing wrong in changing the skies in post-processing, especially if you cannot revisit the same place again. The below tutorial is extremely easy and straightforward. Just follow the steps.
1. Select your photos: you need to have shot many skies and saved them in your stock library for later. Select your target photo (the one you want to change the sky for) and open both in Photoshop. Before your proceed any further create a new layer from the background layer.
2. Copy the sky from the selected photo by selecting the Rectangular Marquee Tool (Shortcut M). Go to the menu - Edit - Copy. Close the sky photo as you don't need it anymore.
3. Go to the target photo and paste the sky over. Your Layers palette should look like this.
4. Add a Black Layer Mask by holding down the Alt key (Option key on Mac) and click on the "Add Layer Mask" on the button of your layer palette (that rectangle with a white dot in the center).
5. Reveal the skies from the layer beneath by selecting a white brush and start painting over. Always remember remember that in Photoshop white reveals and black brush hides on layer masks. If you make a mistake, switch brush colors again (just press X on your keyboard and your brush colors will change from white to black). Your photo should look like this.
6. Apply a Blending Mode to your taste. I like the Lighten for this one with Opacity 95% and Fill 100%. Notice the nice and smooth blending effect from the top of the car roof.
And you're done!