How do I update my iPhone 6s?

Go to Settings/General/About and note the iPhone Model Name, then check its version compatibility from this list:

• iPhone XR, XS, SE 2nd or 3rd edition, iPhone 11 and newer: You can update to iOS 17.4.1
• iPhone 8, and iPhone X you can update to 16.7.7
• Any iPhone series 6S and 7, and original SE: You can update to 15.8.2
• iPhone 5S or 6: You cannot update to iOS 13 or higher; you can update only to iOS 12.5.7
• iPhone 5: You can update only to 10.3.4
• iPhone 5C: You can update only to 10.3.3
• iPhone 4S: You can update only to 9.3.6
• iPhone 4: You can update only to 7.1.2

If your phone is eligible to be updated go to Settings/General/Software Update and follow the instructions.

An iphone 6s can only be updated to iOS 15. (currently 15.8.2)

See how here:

Update your iPhone or iPad - Apple Support

If any problems are encountered, see:

If your iPhone or iPad won't update - Apple Support