Update iPhone 6 iOS

The iPhone 6 cannot be updated to anything higher than iOS 12.5.7.

If you need to run a higher iOS, you need a new phone.

To update to the maximum of iOS 12.5.7, please 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.