Open the Apple Watch app on your iPhone
Tap My Watch
Tap your watch at the top of the screen
Tap the i button (next to the watch)
Tap Unpair Apple Watch
Tap Unpair to confirm
Keep your iPhone and Apple Watch close together until the process finishes
If prompted, enter your Apple ID password to disable Activation Lock
Wait for the watch to finish unpairing, then follow any on-screen steps to set up or store the watch