How to update iPhone 6s Plus to the latest version?

Your profile states that you have an iPhone 6s Plus with iOS 15 installed. You are not going to be able to install a newer version of iOS on that iPhone model. IOS 15 is the final version. This is because your iPhone 6s Plus does not have the necessary hardware to run newer iOS versions efficiently.

Axel F.

You cannot update an iPhone 6s to the latest version of iOS (iOS 18).

For the iPhone 6s, iOS 15 is the end of the line.

You will receive notification when a new update is available. That is the one you can install from official source.