Forgotten password to unpaired watch

If you forgot the watch passcode, see:

If you forgot your Apple Watch passcode - Apple Support

If you forgot your Apple ID password, see:

If you forgot your Apple ID password - Apple Support

If you are the ORIGINAL owner, then see the 'Start a support request' section in:

How to remove Activation Lock - Apple Support

Here are two ways to unpair your watch if you no longer have the paired iPhone:

• 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 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.

Couple sets of steps to take.

First, reset your Apple Watch passcode,

1. Put your watch on its charger and keep it there until you complete these steps.
2. Press and hold the side button until you see Power Off.
3. Press and hold the Digital Crown until you see Erase all content and settings.
4. Tap Reset, then tap Reset again to confirm.

Next we’ll have to unpair your Apple Watch from your iPhone (without the iPhone)

1. On your Apple Watch, tap Settings > General > Reset > Erase All Content and Settings. 
2. Type your password if prompted.
3. For GPS + Cellular models, choose to keep or remove your cellular plan.

• • If you want to pair your Apple Watch and iPhone again, keep your plan. 
  • If you don't want to pair your Apple Watch and iPhone again, remove your plan. If you aren't going to pair with a different watch or iPhone, you might need to contact your carrier to cancel your cellular subscription. 

1. Tap Erase All to confirm. This resets your Apple Watch to the factory settings.

This should let you pair your Apple Watch to your new iPhone.

You can ask apple to remove the activation lock online on this page;

Online Activation Lock Removal.

Or this;

If you can't remember/gain access to the apple id that paired the watch to the phone.Take the watch and it's receipt along with some photo id and any other proof of ownership to an Apple Store and request that they remove the Activation Lock for you.