How can I update my iPhone?

An iPhone 6 Plus can only be updated as far as iOS 12.5.6

Tap Settings

Tap General

Tap Software Update

If the message indicates that your phone is up to date, you have reached the end of the update road. No more updates will be available.

An iPhone 6 Plus isn't capable of 13 or above. To install 12.5.6, use a computer or the Software Update function in Settings.

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iPhone 6 cannot be updated beyond iOS 12 (iOS 12.5.6)