A1586
Why I can’t update my phone version?
iPhone 6, iOS 11
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Top-ranking reply
You should be able to update to IOS 12.
See the Apple Support article(s) below.
iOS 13 and iPadOS are compatible with these devices - Apple Support.
iOS 12 is compatible with these devices.
Get help with over-the-air iOS updates
Update – Can’t or can’t restore
Have you tried using iTunes on a computer or Finder if running Catalina to do the update?
Update software on iOS devices in iTunes on Mac
If you see an error when you update or restore your iPhone, iPad, or iPod
