Why can't I update iMovie?
If you're running a system prior to OS X 10.9, you might consider updating it. Upgrading Apple's iApps from the 2011 versions to the 2013 versions requires either Mavericks (for version 10.0.5) or Yosemite (v10.0.6). The boilerplate message from the App Store has misled a good number of people.

If, on the other hand, you are using 10.9 or 10.10, first check the "Purchases" page of the App Store (Fourth icon across on the top of the toolbar) to see if the iApps are available for download. These versions replace--not upgrade--pre-installed iApps with bundles which tie them to a combination of your Apple ID and your computer's hardware ID.

If such isn't the case, contact Apple Support for Mac Apps, and explain your situation. You are entitled to an upgrade.