How to downgrade my AppleWatch to watchOS 3?

My iPhone SE is broken so I decided to switch back to my previous iPhone 5. To couple my Apple Watch to the iPhone 5 I needed to reset the Apple Watch in advance. After I've done the factory reset I realized that I neither be able to upgrade my iPhone 5 to iOS 11 nor undo the factory reset nor use my Apple Watch just as a regular Watch. Is there a way to downgrade to watchOS 3? It would be a huge disadvantage if you loose your Watch as well, when your iPhone breaks. Best Jussen