Watch os to IPhone 6 IOS 12?

watchOS 6 requires an iPhone capable of running iOS 13. That is an iPhone 6s, SE, 7, 8, X, XS, XR, 11

For now your Apple Watch will continue to run watchOS 5 and provide the same services it has been providing.

When you decide to get a new iPhone, you will be able to update your Apple Watch to a newer version of watchOS

Zavodoff wrote:

if I untie the Watch from my phone, tie it to the iPhone 8, update the watch, and then tie it back to my iPhone 6, will they work with my phone?

You will not be able to pair the watch to your iPhone 6 once it has been updated to watchOS 6.