I want to update my iPhone

The last supported iOS version for iPhone 6 and iPhone 6 Plus is iOS 12. iOS 12.5.7 is the most recent of that.

For apps requiring iOS 13 or later, you’ll need a newer (used, refurb) or new iPhone.

iPhone 6s or iPhone 7 are supported as far as iOS 15, and iPhone 8 and later are needed for iOS 16.

Lucyjef wrote:

How will my phone display if I updated it

In order to check which iOS version is currently running on an iPhone, go to:

Settings>General>About>Software Version

Regards

Giulio