How do I install the latest macOS update on my MacBook Air?

Latest Software would be macOS 15 Sequoia

The Age of this computer will Govern what newer version of macOS it will Qualify to install. 

Find out which macOS your Mac is using

Does this computer appear as Listed and qualifying for an upGrade to a newer version of macOS ? 

Safari will open a redirect and open the Apple Store where the download can commence.

If Yes, suggest having at least 50 GB of Empty Space on the drive.

It will allow the Full Version ( about 12.xx GB ) to download, then expand and then commence to install.

If there is Third Party Security Software installed - remove it.

It will interfere with both the download and installation process.

The below links will help identify which version of macOS  this computer may Qualifies to run.

macOS Sequoia compatible Computers 

macOS Sonoma is compatible with these computers

Always make a Time Machine Backup  before doing any upGrades 

What do you get when you go to the System ➙ General ➙ Software Update settings pane?

Another possible method is to go to the App Store and search for macOS Sequois. If you're MBA is able to run Sequoia you should get a download button for the latest Sequoia version

See this for guidance:

Update macOS on Mac - Apple Support