Enter the passcode on the Apple Watch
If the watch is locked to your wrist, put it on and enter the passcode once
If you forgot the passcode, erase the Apple Watch and set it up again
On the paired iPhone, open the Watch app
Tap Passcode
Turn on Unlock with iPhone
To unlock with iPhone, keep the iPhone nearby and unlocked
If Activation Lock appears, enter the Apple ID and password used to set up the watch