hi, I have a 2018 Mac book air with 'High Sierra' 10.13.6, is this the latest software ? because an installation says its too old . .

Back up your data, check the application compatibility table, click here, and then on the download link for a higher OS.

Catalina and above won't run 32-bit software and split iTunes into several applications; if you choose one of them, use Go64 as well.

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In addition to the advice provided above, note that the 2018 MacBook Air actually shipped with a minimum OS of 10.14.1, so you will need to double check the vintage of the machine.

macOS 12 Monterey is the Best it could run

macOS Monterey is compatible with these computers

Follow this How to get old versions of macOS and verify what version this computer Qualifies to install.

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, 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 Qualify to run.

Computer Qualify for the Monterey upgrade

macOS Big Sur 11 hardware requirements

macOS Catalina 10.15 hardware requirements

Make Time Machine Backup  before doing any upGrades