How do I update my MacBook Air to this 64 bit?

Your MacBook Air is already 64 bit. If you are receiving a message regarding 64 bit compatibility, it is referring to a particular app on your computer. It will not be an issue until the next version of Mac OS X is released.

Your MacBook Air is already 64 bit.

What you need to update to 64 bit are those apps that are issuing the message, and only if you plan on updating to macOS Catalina when its released.

Catalina will not be able to run 32 bit apps.

If the message is being issued only when opening Apple apps, then ignore it, as all those will be updated to 64 bit with Catalina.

For 3rd party apps, contact the developers to find out if they have 64bit updates available.

If you don't plan on updating to Catalina when its released, or your MacBook Air cannot be updated to Catalina, then there's nothing you need to do.