How to update my iPhone to a new iOS version?

The iPhone 6 can be update to iOS 12.5.6 but not beyond that. If yours is not already updated, see instructions in Update your iPhone or iPad - Apple Support

To update to the maximum of iOS 12.5.6, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information.