I want to update May cellphone but how?

Your iPhone 6S can be updated to iOS 15.7.9, no further.

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.7

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.7.9

• If you have an iPhone 8 or higher, or SE 2nd or 3rd generation, you can update to 17

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support

Your tag line indicates that you have an iPhone 6s Plus running iOS 15. That phone can run the latest version of iOS 15 – currently iOS 15.7.9. It cannot run iOS 16 or 17.

About the security content of iOS 15.7.9 and iPadOS 15.7.9 - Apple Support

If you do not have iOS 15.7.9 yet, see

Update your iPhone or iPad - Apple Support