How can I update my IPhone 6 to the later version

You have an iPhone 6 and you are on the latest version, iOS 12, that is available for it. iOS 12 is as far as your iPhone 6 can go.

iPhone 6s and newer are compatible with iOS 13.

Now, your Apple Watch appears to be running on Watch OS6 and therefore you cannot use it with your iPhone 6 with iOS 12. You either need an iPhone that can run iOS 13 OR an Apple Watch OS5 to run with your iPhone 6.

Note that you are not going to be able to downgrade your Apple Watch to WatchOS5. You can take it to an Apple Store to see whether they can do that for you. Long shot but worth a try.

Axel F.

You can't. The iPhone 6 is stuck on iOS 12.4.4 forever. You will have to buy a older watch or newer phone.