On rotten sky image
Magic wand: Top bar : tolerance 15 - 30 ish. Tick anti alias. Tick contiguous.
Defaults to +, equates to add, in second double box from the right in top bar.
If goes overboard, hold alt [this highlights 3rd box from left in top bar] and click to remove.
Quick selection tool. Tick auto enhance in top bar.
Choose brush size for comfort.
Click [ bracket makes brush smaller. ] bracket makes brush bigger
This tool learns as it goes, so click a bit, if goes overboard hold alt and click to remove from selection, then continue til all selected.
Select. Modify. Expand. 3 pixels.
This is not mandatory and will depend on foreground subject, could try without first.
Open nice sky image.
Not vital but easier if in same bit mode [16 or 8] and size as foreground image.
Select all .....Ctrl A. Edit copy.......Ctrl C
You may now close sky image if you wish.
Reopen rotten sky image.
Edit. Paste Special. Paste into................ Bingo !!
To resize or move sky make sure sky layer is highlighted.
Ctrl T [Edit Free transform]
If cannot see handle boxes, Click Ctrl and 0 [nought]
Can then drag/move sky, or change size to please by dragging on corner boxes.
To make sure you maintain aspect ratio, either hold shift while you drag or click padlock between W and H on top bar.
Tick in top bar or click enter when you are done. If you are really good do it on a dupe layer then u can change your mind, and bin it and start again!
If your sky be nice but noisy, zoom in real big and try adding a few px of gaussian blur. I know Paul's way with a brush was more area specific but for me ....well, guess I am less perfectionist !