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To replace the sky in a picture, start by making a selection of just the sky. Here the Magic Wand was used to select the predominantly blue sky region.
Now open a substitute sky picture and make a selection of the sky. Copy the selection (Edit > Copy) and then switch back to the original picture and choose Edit > Paste Into.
Immediately after pasting, press the Ctrl/ Cmd T keystrokes combination and use the Free Transform feature to resize and adjust the proportions of the new sky to fi t the area of the old.