How can I upgrade my iPhone 6s version

You may not be able to update that iPhone. To check, select Settings > General > Software Update. You will see the current iOS version. If an update is available, you will be prompted to do so. If not prompted, its iOS is as high as it can go.

If you mean to iOS 16, you can't. The 6s cannot go beyond iOS 15.x.

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.