I am trying to download iMovie on my MacBook Air but its saying that I need to update my software. when I go look at my updates it says its all up to date and there is no updates available

The current version of iMovie (iMovie 10.2.2) requires at least macOS 10.15.6 Catalina to be installed.

You will not be seeing a software update, if your Mac is running the last version of macOS 10.13. It requires a system upgrade to get to macOS 10.15.6 Catalina.

Can your Mac be upgraded to Catalina? Or is macOS 10.13 High Sierra the last system version compatible with your Mac?

See: macOS - How to Upgrade - Apple (UG)

Hi, Edgar,

Try this procedure to get the highest version of iMovie that your operating system will run.

To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the trash. (You can skip that step if you had iMovie 10 on your computer before, but deleted it.) Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar.   You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash (if you moved it there) and return it to the Applications folder.

-- Rich