Watch Series 3 WatchOS downgrade from 6 to 5 to connect back to my iPhone 6 with iOS 12.4.4

I was using my Apple Watch Series 3 with my iPhone 6 was past 10 months.

I got an used iPhone 7 for my friend. Before giving it to him I tested this phone (iPhone 7). I paired my Apple Watch Series 3 with iPhone 7. After connected to iPhone 7, my Apple Watch Series 3 WatchOS got updated to watchOS 6.1

After completing my friends iPhone 7 testing, I unpaired my Apple Watch from iPhone 7.

After unpairing, I tried to pair with my iPhone 6 (iOS 12.4.4). During pairing process, I got message that my iPhone is out of date and asked to updated iOS. But my current iPhone 6 is up-to date.

After reading in forums, realized that Apple Watch with WatchOS 6 cannot be paired with iPhone iOS 12.

How can I downgrade my Apple Watch from WatchOS 6 to WatchOS 5? So that I can continue to use my watch as I was using for the past 10 months?