I forgot my password to set up my Apple Watch

So, the easiest way to do this is probably by resetting the watch and setting it up again. This will let you set a new passcode but you'll have to reinstall everything and bring back your settings to what they used to be. This probably shouldn't be too much of an issue if you haven't done much to the watch. This will let you set a new passcode you can use to update it.

Here is a support document from Apple that tells you how to use your phone to reset the watch:

https://support.apple.com/en-us/HT204567

Start here;

If you forgot your Apple ID

If you forget your Apple Watch passcode

I think you can also restore from an Apple Watch backup after you reset it, which is usually on your iPhone, you can set a new passcode. I'm not sure though.