What is your current Aperture version? And how did you buy Aperture originally?
If you bought it from the App Store, you should see it in the "Updates" tab of the App Store window; if you bought it differently use "Software Update" from the "Apple" menu.
Software update will only work, if Aperture is installed in the Applications folder, not a subfolder of this folder, and if it has not been renamed. It must simply be called "Aperture".