How do I update my MacBook Air version 10.13.6?

Since you're running High Sierra, that implies that your MacBook Air has a hardware model year somewhere in the range of 2010 – 2017.

If you have a Late 2010 or Mid 2011 MacBook Air, High Sierra is the end of the line. No more updates are available.

If you have a Mid 2012 MacBook Air, you can update to Catalina, but no further.

If you have a Mid 2013 or Early 2014 MacBook Air, you can update to Big Sur, but no further.

If you have an Early 2015 or a 2017 MacBook Air, you can update to Monterey, but no further.

What model / year of Mac do you have exactly? Apple Menu ➜ About This Mac

That will determine what you can update to if you can update at all.

Start by clicking here ➜ How to download and install macOS - Apple Support