How can I update my iOS?

Michaeljames20 wrote:

I want to update my iPhone 6s to iOS 16.4

Sorry, but that is NOT possible. Your iPhone is too old and lacks the hardware required to run anything higher than iOS 15. If you need a higher level iOS version, then you'll have to retire your very old iPhone and buy a newer, more modern iPhone.

iPhone 6s only supports iOS 15, so this is not possible.

Oichoooooo wrote:

iPhone 6s only supports iOS 15, so this is not possible.

The OP's tag line indicates that he or she has an iPhone 6 Plus (no 's'). iOS 12 is the end of the line for the iPhone 6 (no 's') and 6 Plus (no 's').

The iPhone 6s and 6s Plus are more recent, and for them, iOS 15 is the end of the line. That would presumably be enough to run the current Facebook app, which requires iOS 13.4 or higher. But it's not relevant to the OP, unless the OP is thinking of buying a used iPhone 6s or 6s Plus.

I want to update my iPhone 6s to iOS 16.4

Despite what you want, it is not possible for the iPhone 6s to run iOS 16.