How to Replace a photo’s background
Having opened the image that needs its sky brightening up, you need to make a selection. A selection masks off any areas of the image outside its borders, so any changes you make won’t affect them.
Make a selection
Using the Smart Selection tool in the Selections persona, brush over the subject of your photo — the bit you want to keep. Use a smaller brush for the edges and make sure Snap to Edges is selected.
Polish the selection
Use Refine Selection (at the bottom of the left toolbar) to fit your selection outline tightly to the subject’s edges. When done, tap the check mark in the tool’s options, then the three dots (top left) and pick Invert Selection.
Pick a new image
Now your background is selected, replace it by switching to the Photo persona, tapping the three dots again and choosing the Place command. Your new image won’t appear until you drag out a box for it with your finger.
The new sky image will go on a new layer on top of your original, so use the Layers palette to move it behind. It will disappear, but select the new top layer and use the three dots > Pasteboard > Cut to make the old sky vanish.
Tweak and share
With your sky replaced, your subject is still on a separate layer, and can be moved and scaled. When you’re happy with the picture, tap the icon of a document with three dots inside it (top left), then tap Export and save the picture.