upgrade mac os to 10.14

Unless your Air is a mid-2012 or newer model, you are done at High SIerra.

You can confirm this from the menu > About This Mac panel, and the content in parenthesis on the product description line. If it is a mid-2012 or newer model, then using strictly Safari, visit how to get old versions of macOS, and click the Mojave link that will take you to the Mac App Store, where you can download it. Don't forget to perform a Time Machine backup before the upgrade.

If your Mac qualifies as new enough...

macOS Mojave 10.14.6 - Technical Specifications

General Requirements

OS X 10.8 or later

2GB of memory

12.5GB of available storage (OS X El Capitan 10.11.5 or later)*

Some features require an Apple ID; terms apply.

Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Mojave:

MacBook (Early 2015 or newer)

MacBook Air (Mid 2012 or newer)

MacBook Pro (Mid 2012 or newer)

Mac mini (Late 2012 or newer)

iMac (Late 2012 or newer)

iMac Pro (2017)

Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)

Don't use FireFox for these links…

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12