I what to update my iPhone 6

You have an iPhone 6 with iOS version 12 installed (according to your profile). That is the highest version of iOS that you can install on your iPhone 6. You will need a newer model iPhone to use higher versions of iOS.

Axel F.

It's not capable of 13 or above. To update it to 12.5.6, use a computer or the Software Update function in Settings.

(225925)

Please read the responses provided to you above. Thanks.

Axel F.

An iPhone 6 can only be updated at an Apple Store or authorized reseller. Bring several hundred dollars (or your local equivalent) with you,

How to update my iPhone 6 plus

I can't afford to buy a new one what I have to do to to retain the iOS capacity of my unit

You can't.

An iPhone 6/6plus can't be updated beyond 12.5.6, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.