Here are two ways to unpair your watch if you no longer have the paired iPhoneand will let you use your old App[le ID & password:
- See the 'How to erase your Apple Watch if you don't have your iPhone' section of:
Unpair and erase your Apple Watch - Apple Support
- With a web browser, go to: https://www.iCloud.com and sign in with your old Apple ID. Open Find My iPhone and select All Devices (at the top). Then click your Apple Watch, select 'Erase Apple Watch', and select Next until the device is erased. Finally click "X" next to remove your Apple Watch.
If for some reason those don't work and If you are the ORIGINAL owner, then see the 'Start a support request' section in:
How to remove Activation Lock - Apple Support