I want update my iPhone 6 Plus 12-14

There is nothing to solve. That cannot be done. The iPhone 6 Plus does not have the necessary hardware to support an iOS higher than the iOS 12.5.5 that it currently supports. If you have a need for an iOS higher than 12.5.5, you will need to purchase a newer model iPhone.

iPhone 6 Plus is an outdated iPhone (9 september 2014), can’t be update to iOS 13 and higher.

iOS 12.5.5 is last security updates for iPhone 6 Plus, -> released September 23.

Go to Settings -> General -> Software Update -> to update to iOS 12.5.5