How to unpair an Apple Watch from a broken phone
My Apple Watch is paired to a phone that I can’t get into to impair it, how am I able to pair it to my new phone?
Apple Watch Series 1, watchOS 6
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To unpair and erase watch and remove Activation Lock.
On a computer, go to iCloud.com and sign in with your Apple ID.
Go to Find Iphone.
Select All Devices, then click your Apple Watch.
Click Erase Apple Watch.
Select Next until the device is erased.
Click the remove button next to your Apple Watch.
Then go here;
How to pair your Apple Watch with a new iPhone – Apple Support