Q: Aperture needs update, but doesn't appear in the app store anymore

Please search the forums, this issue has been asked and answered numerous times.

Why not provide the info instead of telling him to search the forum. It's frustrating enough that apple made aperture useless

Have you found the update by now? If not:

You need to update to Aperture 3.6, that is the compatibility update for Yosemite.

The problem is that Apple stopped selling Aperture, so you will neither find an update on the Updates page of any App Store nor can you purchase Aperture any longer. If you try to launch an incompatible Aperture version, you will receive an update message sending you on a wild goose chase.  The only way to update is by reinstalling Aperture, if you already own Aperture and your purchase is associated with your AppleID at the Mac App Store.  So look at your Purchases history, as described in this user tip:

Updating to Aperture 3.6 After the Release of Photos

Which version of Aperture do you have? If your Aperture version is not an App Store version, you will need the help of the App Store Support for updating.

The new Photos.app can open Aperture Libraries, but is in no way a successor for Aperture, it barely suffices as a successor for iPhoto (see:  Photos vs iPhoto:  Features and Capabilities).

There is not yet any Apple application to replace Aperture, see:  How Photos handles content and metadata from iPhoto and Aperture - Apple Support

Why not provide the info instead of telling him to search the forum. It's frustrating enough that apple made aperture useless

And, the reason for you not telling him/her is?

Forum 101: https://discussions.apple.com/docs/DOC-5951

And, Aperture is not useless, I'm still using it just fine.