How to update iPhone 6s iOS version?

An iPhone 6s, iPhone 6s Plus, iPhone 7, iPhone 7 Plus, or iPhone SE 1 isn't capable of 16 or above; they require an A11 or better CPU. To install 15.8.3, use a computer or the Software Update function in Settings.

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Your iPhone 6s Plus is capable of running up to iOS 15.8.2, no more. I can see your iPhone is currently running iOS 13, so you can simply update by going to Settings —> General —> Software Updates, then just update your phone to 15.8.2, you’re all set!

Goku3508_iOS wrote:

I’m still learning the ecosystem, My answers aren’t always accurate.

About iOS 15 Updates - Apple Support

"iOS 15.8.3" would be correct as of today. "Latest release of iOS 15" would also cover it, and would age better if Apple decided to release an iOS 15.8.4. Or you could word it as "the iPhone 6s cannot update to iOS 16 or later." That also would age well.

Background:

As far as I can tell, iPhones can almost always update to the latest release of the latest major version of iOS that they support. There was one case in which "latest version of iOS 10" would have been incorrect: the iPhone 5c could only update to iOS 10.3.3, whereas other iPhones for which iOS 10 was the end of the line could update to iOS 10.3.4. That was the exception which proves the rule.

.......just update your phone to 15.8.2, you’re all set!

You mean iOS 15.8.3, which is the current version of iOS 15.

I’m still learning the ecosystem, My answers aren’t always accurate.

Goku3508_iOS wrote:

I’m still learning the ecosystem, My answers aren’t always accurate.

It's very easy to check things like that with a little bit of internet searching. If you're not sure, check.

Google is restricted (I did) , I only have access to apple sites now, I'm only replying with what I know