If you have a great photo but the sky is not that great you can replace it for another one from another photo. I have done this many times and can make a big difference:
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Open the photo that needs another sky. Select the sky.
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Open the photo with a nice sky. Select the sky from the second photo. Press Ctrl+C to copy the nice sky.
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Switch back to the original photo (the sky should be still selected).
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Create a New layer. Then, click onĀ Edit and Paste Into. Now you have the new sky instead of the old one.