Update

You can update your iPhone to iOS 15.7.3 by going to Settings -> General -> Software Update and installing the particular update.

Make your iPhone has

• charged to at least 50%
• preferably connected to the charger
• disabled any VPNs
• connected to a fast, reliable Wi-Fi

However, your iPhone cannot update to iOS 16 due to your internal hardware being too old to support it.

The iPhone 6s is over 7 years old.

iPhone 8 and later models are capable of supporting iOS 16.

Here is a list of unsupported iPhones with max iOS supported:

Model            Architecture  Introduced       Max supported iOS
iPhone (1st)     32-bit        June 2007        iOS 3.1.3
iPhone 3G        32-bit        July 2008        iOS 4.2.1
iPhone 3GS       32-bit        June 2009        iOS 6.1.6
iPhone 4         32-bit        June 2010        iOS 7.1.2
iPhone 4S        32-bit        October 2011     iOS 9.3.6
iPhone 5         32-bit        September 2012   iOS 10.3.4
iPhone 5C        32-bit        September 2013   iOS 10.3.3
iPhone 5S        64-bit        September 2013   iOS 12.5.7
iPhone 6         64-bit        September 2014   iOS 12.5.7
iPhone 6 Plus    64-bit        September 2014   iOS 12.5.7
iPhone 6S        64-bit        September 2015   iOS 15.7.3 <- Your iPhone model
iPhone 6S Plus   64-bit        September 2015   iOS 15.7.3
iPhone SE (1st)  64-bit        March 2016       iOS 15.7.3
iPhone 7         64-bit        September 2016   iOS 15.7.3
iPhone 7 Plus    64-bit        September 2016   iOS 15.7.3