Apple stopped supporting Aperture in about March 2015, when they introduced the Photos app.
If you purchased Aperture 3.4.5 through the App Store using your Apple ID you may still be able to get version 3.6.
First of all you need to back up your Mac. Once the back up has finished open your Applications folder and copy the Aperture.app to an external disk or USB. Now move the one in your Applications folder to the trash and delete it. Now disconnect any external disk USB stick etc that may have any incidence of the Aperture.app on it, leave the Aperture Library alone. Restart your Mac open the App Store and see if you get offered Aperture in the Purchased section, make sure it is version 3.6.
If you are successful and do manage to download version 3.6 when you open it it will convert your Aperture Library to be compatible with 3.6, this is irreversible. If 3.6 does not show in the Purchased section then you can just put back the 3.4.5 version you made a copy of earlier.
Remember you can only do this if you purchased version 3.4.5 through the App Store using the Apple ID associated with your Mac.