How can I update my iPhone 6s?

Your iPhone 6 can be updated to: iOS 12.5.7

You can check your current version here:

Settings → General → About

You can download and install software updates here:

Settings → General → Software Update

Remember to connect to Wi-Fi and a charger. Your iPhone must have more than 50% power to

update without a charger connected.

You can find more useful information here:

Update your iPhone or iPad - Apple Support

If you want to update your iPhone via a PC or Mac, see here:

• Mac: Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

• PC (iTunes): Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

The 6s is now a very old phone, it cannot be updated to the latest iOS, only to iOS 12.5.7.