Here is how to reset the Apple Watch passcode: If you forgot your Apple Watch passcode - Apple Support
The passcode reset will reset the device, but will not clear Activation Lock.
To clear Activation Lock, you’ll need to recover the Apple ID and password, or proof of original purchase.
See Activation Lock, here: How to Unlock an iPad or iPhone? - Apple Community
If the paired iPhone is no longer available, see that section here: Unpair and erase your Apple Watch - Apple Support
Series 2 is from 2016 to 2027, and support tops out at watchOS 6.
If the battery hasn’t been replaced, it’s probably due for replacement.