Updating iPhone 6 plus

I recently got an Apple Watch 5 and am trying to pair it with my iPhone 6 Plus, however my phone needs to be updated to iOS 13 first. Whenever I go into updates it says that my phone is up to date. I have tried turning it off then on, restarting my internet router, and resetting my network settings and it is still not showing an available update. How can I find this update so I can use my new watch?