I want to upgrade my iPhone to iOS 12.7
I want to upgrade my iPhone 6 Plus to iOS 12.7
iPhone 6 Plus, iOS 12
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.