Please update my iphone6s plus software

They iPhone 6s Plus is capable of running the latest iOS release, iOS 15.5.

See here instructions on how to upgrade: How to update to iOS 15 or iPadOS 15 - Official Apple Support

If you have the iPhone 6Plus, without the S behind 6, the latest iOS your phone can run is 12.5.5 due to hardware limitations

See the articles below.

Update your iOS device.  -

Get help with over-the-air iOS updates  -

Update – Can’t or can’t restore   -

Have you tried using iTunes on a computer or Finder if running Catalina or later to do the update?

Update software on iOS devices in iTunes on Mac. -

Phone, iPad, or iPod touch - Update with Finder.  *

iPhone, iPad, or iPod touch - Update with iTunes.  *

If you see an error when you update or restore your iPhone, iPad, or iPod    *

Updating iOS: common issues and solutions.  -

ASC YouTube for  how to update your iPhone, iPad, or iPod touch to the latest version of iOS or iPadOS —wirelessly or using your computer.