I want to update my phone

It is not possible, 

iPhone 6 is an outdated iPhone (9 september 2014), can’t be update to a higher iOS.

iOS 12.5.5 is the last security updates for iPhone 6, -> released 23 september 2021.

Your seven year old iPhone can be updated to iOS 12.5.5. If you want to use features or apps that require iOS 13 or later, you will need to purchase a newer model iPhone.