is your main system language set to English? If not, change the language to English in the "Language&Region" System Preferences (see: Re: I bought Aperture in a box et can't upgrade from 3.4.5 to 3.5.). Then sign back into AppStore and reload the Updates panel.
Other things that could prevent the update are, if you moved Aperture from the Applications folder or renamed it. Then put it back.