I cannot update to watchOS 6 because my iPhone is a model 6 plus and cannot be updated to IOS 13. Is there any other way to update my iWatch?

There are people which have posted in these forums where they managed to get watchOS 6 on their Apple Watch, and then could no longer use it because it would not re-pair to their iPhone 6 (or 6 Plus). Strongly suggest you learn from their mistakes.

A used iPhone 6s Plus would be the least expensive way to move forward. Or just wait until you decide it is time to replace your iPhone 6 Plus with a new model of any kind.

If you update your watch, it will no longer work with your iPhone 6.

You need to get a newer phone capable of running iOS 13 or stick with the version you have.