Am using iPhone 6

To update to the highest possible of iOS 12.5.7, please 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.

Hello ~ You will not be able to update your iPhone any further than iOS 12.5.7. You need a newer device for iOS 14. If you choose a second hand device please click on these helpful links below to make an informed decision…

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Also:

The All Too Common SAD Reality of Buying a Used iPad/iPhone

~Katana-San~

Use a computer or the Software Update function to install 12.5.7. If you encounter a specific issue, describe it in this thread.

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