Update 12.4.5 not on the list
Updating of my iPhone 6
Kalculus Said:
"Update 12.4.5 not on the list: Updating of my iPhone 6"
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Updating an iPhone 6 Plus Past to iOS 12:
A. You can't.  The iPhone will only install to iOS12. See here: Update your iPhone or iPad Software if you're Experiencing Issues with Location, Date, and Time - Apple Support. And, for the note, even if you could update, only the latest version will install.
So, what to try...
B. Update Using a Computer:
Have a Computer Handy? If so, try using a computer to update. Go here: Update your iPhone Using a Computer - User Tip. If any updates are availabe, they will install, sure enough.