Why I can’t update my iso

An iPhone 6 can't be updated past IOS 12.5.4.

See the article(s) below. 

iOS 14 and iPadOS 14 Compatible Devices

iOS 13 and iPadOS are compatible with these devices - Apple Support.    

If you can't upgrade to 12.5.4, see the articles below.

Update your iOS device.        

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

If you see an error when you update or restore your iPhone, iPad, or iPod      

Updating iOS: common issues and solutions

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