How can I upgrade my iPhone by my self

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IOS upgrade or some other upgrade?

An iPhone 6 can't be updated past IOS 12.5.6.

See the article(s) below. 

iOS 16 compatible devices

IOS 15 and iPadOS 15 Compatible Devices.  *

iOS 14 and iPadOS 14 Compatible Devices.  *

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

It depends on what phone you have. If you have an iPhone 8 or newer and want to update to iOS 16, go into Settings, then General, then Software Update. It should show iOS 16. You need to make sure you have enough storage on your device to download the update, but if you do, just click I believe "Download Update" and you should be good to go. It will need to download the update first, and then it will actually update your iPhone. Hope this helps! :)