How to download
iPhone 6s, iOS 12
Question marked as
Your post says you have a 6S, if that's true then you can upgrade to iOS 15.5.
If you have an iPhone 6, it can't be updated beyond 12.5.5, it doesn't have the hardware needed to run a higher iOS.
Go to Settings > General > About and see what model you have.
If you do have a 6S, you will probably need to update using your computer as you are several versions behind.
Update using your computer -
Update iOS on iPhone - Apple Support