Unable to update iPhone 6s Plus

Hi, and welcome to the Apple (user-to-user) Community!

Re: Please I can’t update my phone : i found it difficult to update my phone

Wondering just how you tried to update your iPhone, and what happened ... eg: an error message ?

Seems iPhone 6s Plus cannot update past the iOS 15 versions.

See if you can: Go to Settings > General > Software Update and download the latest compatible iOS version

About iOS 15 Updates - Apple Support says iOS 15.8.1 is that latest version.

If the update doesn't appear on your device, try using your computer to update your device manually.

Learn how to update your device manually

if you're using a Mac with macOS Catalina or later,

or

if you're using a Mac with macOS Mojave or earlier or a Windows PC.

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Trouble-shooting : What to do: If your iPhone or iPad won't update - Apple Support

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More support if needed: iPhone Support Website : (info / support / contact options etc.)

(There's a "Get Support" link (online chat / phone / email etc.) at the bottom of the webpage)

iPhone - Official Apple Support

All the best :-)