I can not update my iOS 12.5.6

8 years is a long time in technology. Most smartphones don’t last even as long as yours has; Apple phones have about the longest useful life of any in the smartphone category; Android phones stop getting updates after about 3 years. You need a 6S or later for iOS 15, and an iPhone 8 or later for iOS 16.

Your phone is the oldest phone that will still work as a phone after this year in the US and most of the rest of the world, as carriers are turning off their 3G networks. Your phone still supports 4G and LTE, and will work as a phone for several more years. A lot of 3rd party apps not longer work on iOS 12, but some still do, and all Apple apps do.

Agbaa wrote:

But I can’t even download Google Photo on it

That’s true, Google has made the decision that they will only support iOS 15 and later. But that is Google’s choice; Apple would not prevent any app from supporting iOS 12 if the app provider chose to.

This is expected behaviour, as the iPhone 6 Plus is not compatible with iOS 13 or higher.

In fact, Apple has categorized this 8+ year old iPhone as a vintage device.

It’s not useless. But its capabilities are severely limited.

Did you just buy this phone ?

If you jut bought it, return it and get your money back. You bought an antique that is no longer supported by most app developers.