For my Apple Watch

1. From the Watch face screen, press the Digital Crownto access the Apps screen.
2. 
3. Tap the Settings icon .
4. Tap Passcode.
5. Tap Change Passcode.
6. Enter your current passcode then enter the new one.
7. To confirm, re-enter your new passcode.

This link may be helpful:

If you forgot your Apple Watch passcode - Apple Support