I am unable to update mac os

The model year of a MacBook Air is available at Apple menu > About This Mac.

A mid 2009 MacBook Air can be upgraded to El Capitan 10.11.6 but no later version. See: How to download and install macOS - Apple Support

A late 2010 through mid 2011 MacBook Air can be upgraded to High Sierra 10.13.6 but no later version. See: How to download and install macOS - Apple Support

A mid 2012 MacBook Air can be upgraded to Catalina 10.15.7 but no later. See: How to download and install macOS - Apple Support

A mid 2013 and early 2014 MacBook Air can be upgraded to the latest version of Big Sur macOS 11 but no later. See: How to download and install macOS - Apple Support

An early 2015 and a 2017 MacBook Air can be upgraded to the latest version of Monterey macOS 12. See: How to download and install macOS - Apple Support

A 2018 or later MacBook Air can be upgraded to the current version of Ventura macOS 13. See: How to upgrade to macOS Ventura - Apple Support

Be aware that if you upgrade to Catalina 10.15 or later you will also need to update any non-Apple 32-bit apps. See: https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html

FoxFifth wrote:

A mid 2009 MacBook Air can be upgraded to El Capitan 10.11.6 but no later version. See: How to download and install macOS - Apple Support

We know that the OP's MacBook Air isn't that old, because it is running Sierra now.

A 2018 or later MacBook Air can be upgraded to the current version of Ventura macOS 13. See: How to upgrade to macOS Ventura - Apple Support

We know that the OP's MacBook Air isn't that new, because none of those MacBook Airs can run Sierra.

That's true, but I suspected that the OP could follow the directions on how to find the model year of his MacBook Air and use that to select his model from the list provided.