My Iphone cannot install the new update

Correct. The iPhone 6s cannot update past iOS 15.7.3. That is the most current version it supports.

If your iPhone is running anything less that iOS 15.7.3, go to Settings ➜ General ➜ Software Update.

If there is an issue with the update there, please explain what that is.

You may need to update using a computer instead. click here ➡️ Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

What happens when you attempt to update?

Any error message?

Which device?

What is the existing version of iOS?

Updating iOS: common issues and solutions - Apple Community