A Simple GIMP 2.8 Tutorial
Have you ever had a nice outdoor picture of something, say a building, where the exposure on the building was good, but the sky looked drab, washed out, or overexposed, and you wondered what you could do to make the sky look better? You could replace the sky either with a simple solid blue color of the appropriate intensity or with a linear or radial blue gradient. Sometimes that's fine, but sometimes you'd like a different effect to make the image more interesting.
In this tutorial I'll describe a technique you can use to create this image:
from this image:
Adjusting the Image
The first thing I did was use Image > Scale Image to scale the image down to 3000 x 2000 pixels. Then I used the Rectangle Select tool  and Image > Crop to Selection to remove distracting and unwanted parts of the image from the sides and bottom. (CTRL+SHFT+A to remove the selection area afterward.)
Next I duplicated the layer using the layer duplication icon  at the bottom of the Layers dialog and, with the top layer selected, chose Colors > Curves and made ajdustments  until I was happy with the result. I like to have the original image layer available when I do this so I can flip back and forth for comparison by clicking on the eye  to hide and unhide the top layer. Once I was happy, I deleted the lower layer by right-clicking on it and choosing Delete Layer.
I decided the image could use a little more sky so I selected Image > Canvas Size and added 100 pixels to the Height field  and also put 100 in the Offset Y field  so that the pixels would be added at the top of the image. Clicking the Resize button  created the new canvas size with a transparent rectangle appearing at the top of the image.
Replacing the Sky
In order to remove the sky, it's necessary to have a transparent alpha channel, so I chose Layer > Transparency > Add Alpha Channel. Then I selected the Fuzzy Select tool  and clicked on the sky to select it . I did not use the Select by Color tool because it also selects pixels in the stripes in the street and the street signs. The Fuzzy Select tool selects an area contiguous to where you click. Since the sky in this image is essentially all one off-white color, a low Threshold  setting ensured that the entire sky was selected with one click. If the sky had been several shades of white, gray, or blue, I would have either turned up the Threshold or turned on the Mode Add to the current selection  and clicked on each color until the entire sky was selected. I had Antialiasing  checked so there would be a little fade along the edge of the selection. After selecting the sky, I hit the Delete key to remove it, then CTRL+SHFT+A to remove the selection.
Now it was time to add the new sky. I clicked on the blank paper icon  at the bottom of the Layers dialog to create a new, full canvas-sized layer filled with Transparency and moved this new layer below the image layer. (Left-click and drag down.) I chose a light blue (hex code bce0ff) from another photo and used the Bucket Fill tool to fill the layer . Then I created a second new layer, placed it between the image and the light blue layer, and filled it with a darker blue (hex code 89bdf2) .
With the darker blue layer selected, I chose the Ellipse Select tool  and dragged out an elliptical selection . Then I chose Select > Feather , entered an estimated likely number , hit OK, hit the Delete key, then CTRL+SHFT+A to remove the selection. The best number to use for feathering will vary depending on the image resolution and the size of the selected area. I tried several values before finally settling on 300. Remember, CTRL+Z is your friend.
Notice how, in the final image, the curve of the gradient in the sky seems to follow the curve of the building. This new sky has enhanced the image and added a level of interest that was otherwise lacking.
References: This tutorial was inspired by an article entitled The Real Estate Photographer's "Perfect Sky" by Scott Kelby and Felix Nelson in the March 2004 issue of Photoshop User.