Press and hold the current watch face on your Apple Watch
Swipe left or right to choose a different watch face
Tap the watch face you want to use
Press and hold the current watch face again
Swipe to the far right and tap the plus button to add a new watch face
Turn the Digital Crown to browse available watch faces
Tap the watch face you want
Tap Add
To change the wallpaper on a paired iPhone, open the Watch app
Tap Face Gallery
Choose a watch face
Customize the style, color, and complications
Tap Add to set it on your Apple Watch