I've spent 3 hours trying to upgrade my OS. I've gone to the Apple icon, clicked on software update and it says I'm up to date. Currently my OS is Yosemite. I'm trying to upgrade my macbook air to Mojave 10.14.6 OS so I can download imovie.

Hi, u7,

See if this link helps re downloading Mojave:

https://support.apple.com/en-us/HT210190

-- Rich

The link posted before is a great help, but I just wanted to add, if you update from Apple icon it will make updates to the current macOS, for instance Yosemite is 10.10, so you will update to 10.10.1, 10.10.2, 10.10.3 and so on, when you want to update the entire macOS from Yosemite to El Capitan, Sierra, High Sierra, or Mojave you have to go into the App Store and download the current version or latest version your Mac can run, so at this point you will be going from 10.10 to 10.11, 10.12 and so on. This meas, the first numbers 10.10 is the name of the version Yosemite, and in your case Mojave would be 10.14, the last numbers are the actual updates for that OS running on your Mac, so with Mojave there were 6 updates going up to 10.14.6

Does this make sense?