Open the Settings app
Tap Wallpaper
Tap Add New Wallpaper or Choose a New Wallpaper
Select a wallpaper from Photos, People, Photo Shuffle, Emoji, Weather, or Collections
Adjust the image by pinching or dragging if needed
Tap Add
Choose Set as Wallpaper Pair, Set as Home Screen, or Set as Lock Screen
If using a photo, tap Done if prompted
To change it later, repeat the same steps