When downloading updates in the Mac App Store, it’s a simple thing to start and pause your download. Make sure you’re logged into the correct iTunes account for the specific update, and hit the dark gray Update button when you’re ready. If you want to pause the download, simply hit the same button, which now says Pause.
But what if you want to completely cancel the download and start again later?
Luckily, this is a pretty simple thing to do.
When in the App Store, click on the Update button to start the update process. If you want to cancel the download completely, simply hold down the Option key, which will change the Pause button into a Cancel button. Click on Cancel and the update will stop downloading, no matter where it is in the process (unless it says Installing; in which case, you’re out of luck).
The update will still show up in your Updates tab in the App Store, but it will just start over from the beginning, unlike Pause, which will resume wherever it was in the process of downloading the update.
This could come in handy if you just want to quit an update for any reason, but don’t want to resume it in the near future.
Via: Macworld Hints