That will be tricky, if you already upgraded your Aperture library to version 3.4.1.
- You need to restore an earlier version of your Aperture Library from your backup.
- You need to restore an earlier Aperture version.
How did you buy Aperture? Do you have installation media or a serial number for Aperture 3?
And do you have a TimeMachine backup of your Aperture Library or a vault?
What is the problem with your current Aperture Library? Maybe it will be easier to fix your current system then to downgrade.