I don't know how to update my MacBook Air

Hello Ajna1111,

We'd like to provide you with the steps that can assist you with updating your MacBook Air:

Update macOS on Mac

"Use Software Update to install updates and upgrades for macOS and its built-in apps, including Safari.

Before installing new software, it's a good idea to back up your Mac. If you have an external storage device, you can use it to back up your Mac with Time Machine.

Use Software Update

To check for new software, open Software Update. It's in System Settings (or System Preferences), which you can open from the Apple menu  in the corner of your screen:

• Choose Apple menu  > System Settings. Click General on the left side of the System Settings window, then click Software Update on the right.
• Or choose Apple menu  > System Preferences, then click Software Update.

If Software Update finds new software, click the button to install it. You will be asked to enter your administrator password. During installation, your Mac might show a progress bar or blank screen several times, and you should avoid putting it to sleep or closing its lid.

If Software Update says that your Mac is up to date, then no new software is currently available for your Mac model. Software Update shows only software that is compatible with your Mac. 

If you don't see Software Update

If you don't see an item named Software Update in System Settings or System Preferences, click the Spotlight icon  in the corner of your screen, then type Software Update in the search field. From the search results, choose Software Update.

If your older Mac doesn't include Software Update, use the App Store app instead. It's in your Applications folder. Click the Updates tab in the App Store to check for updates, or search for software by name.

If you need help

• If your Mac doesn't have enough storage space to install
• If your Mac stops at an Apple logo or progress bar during installation
• If an error occurs while downloading or installing
• If you want to download an older compatible version of macOS
• Find out which macOS your Mac is using
• If you want to update your iPhone or iPad"

Thanks for reaching out to Apple Support Communities.

Take care