How to update iPhone software?
how to update software
[Re-Titled by Moderator]
iPhone 6, 12
Assuming you have a compatible device, 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.
NOTE: the iPhone 6 can be updated to iOS 12.5.6, no further.