I have an Apple watch 4 and a Iphone 6. How can I update my Watch without buying a new phone?

You cannot. watchOS 6 requires iOS 13 which an iPhone 6 cannot run.

But your Apple Watch will still work the way it is with your iPhone 6 on iOS 12. Everything you were able to do yesterday, will continue to work tomorrow.

And when you decide to get a new iPhone, you will then be able to upgrade your Apple Watch.

I am a user like you, so I didn't kill anything.

And your iPhone still works the way it did yesterday, and your Apple Watch works the same as it did yesterday. Nothing is dead.

If you wish to tell Apple then send Apple Watch Feedback

https://www.apple.com/feedback/watch.html

They will not reply, but it does get read.

You’ve killed two devices with the iOS 13, my iPhone 6 (understandable, it’s old), and my series 3 watch (infuriating because it isn’t old!!)

Apple needs to remedy this for the series 3 owners