how to install software updates on MacBook Air?

If by "Software Updates" you mean macOS version updates and your Mac is running macOS 10.13 High Sierra or earlier, there will not Software Update option in System Preferences.

The Software Update option first appeared with macOS 10.14 Mojave.

To get a new macOS version see here: link-> How to upgrade to macOS Monterey - Apple Support

If your Mac does not support Monterey, then see here to get an older version it can support: link-> How to get old versions of macOS - Apple Support

Click on Apple in top left and pick updates. Also you should get notified with the app store app if any app updates in your dock