IPhone 6 update

On your iPhone......

Tap Settings

Tap General

Tap Software Update

If you have iOS 12.5.6 you already have the most up to date version for the 8 year old iPhone 6. It cannot be updated any further.

If you are asking about iOS 16, you will need at least an iPhone 8 or higher.

If you meant the latest version it can support:

Update your iPhone or iPad - Apple Support

If you meant something later than iOS 12, the iPhone 6 can only be updated to iOS 12.5.6:

Apple security updates - Apple Support

An iPhone 6 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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The iPhone 6 is not compatible with iOS 13 or higher iPhone.

It can be updated to iOS 12.5.6, no further.