Why can’t I update when it needs an update
Why can’t I install and download my software update
iPhone 6 Plus, iOS 12
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IOS update? Are you trying to download IOS 12.5.5 or another update? An iPhone 6 Plus can't be updated past IOS 12.5.5.
See the articles below.
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 or later to do the update?
Update software on iOS devices in iTunes on Mac. -
Phone, iPad, or iPod touch - Update with Finder. *
iPhone, iPad, or iPod touch - Update with iTunes. *
If you see an error when you update or restore your iPhone, iPad, or iPod *
Updating iOS: common issues and solutions. -
https://www.youtube.com/watch?v=csJAEyvvruY