how to change my iWatch passcode?

Did you forget the passcode. If that is the case, see this, If you forgot your Apple Watch passcode - Apple Support

If you just wish to change your passcode, go to the Watch app on your paired iPhone, go to Passcode>Change Passcode.

Firstly, it is Apple Watch, not iWatch. Hope you get it correct. You can do it directly on the watch by going to settings, scroll down to passcode and tap on change passcode. Follow the instructions and you are good. Alternatively, go to the watch app on your iPhone while connecting to the watch. Scroll down and find `Passcode`, tap on that and go to change passcode. Follow the instructions there. Hope this helps!

On your iPhone> Watch App > Passcode> Change Passcode

If however, you forgot your passcode, the Related Article link which brought you here provides the directions will need to follow to erase your watch and set it up again.