How to upgrade IPhone 6s to IOS 18?

You can't.

An iPhone 6S can't be updated beyond 15.8.3, it doesn't have the hardware required to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

• 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 15.8.3

• If you have an iPhone 8 or X, you can update to 16.7.10

• If you have an iPhone SE (2nd generation or later), or XR through 16, you can update to 18.3.1

A 6s can only run up thru iOS 15. (currently 15.8.3)