how do I update my Safari on the MacBook Air

That is because they don't exist. Safari, is installed as part of the operating system, normally it cannot be individually upgraded. it is periodically updated when the computer's operating system is updated or upgraded. If your problem is being unable to access certain web sites, it is because web site engineers write their software to work with specific browsers and specific versions of them. For some reason, they often choose to not be compliant with Safari. You can hope that your web site's developers eventually update their site to work with Safari, or you can use another browser that does work, such as Firefox, Oprah or Chrome if you are unable to upgrade your operating system.

The only Safari updates are for currently supported operating systems: 1) Mojave, 2) Catalina, and 3) Big Sur. If you have an Apple operating system older than these, your version of Safari has no update available.

Safari 11.1.2 is the highest Safari version you can get on macOS 10.11 El Capitan.

If your Mac is a supported model, you may be able to update to a newer macOS version and get a newer Safari version along with it.

What Mac model do you have exactly? Go to Apple Menu->About this Mac. Post the the Year and x,y identifier.