This release addressed many community complaints by reintroducing "pro" features that had been removed in the initial transition to version 10. The Garageband Guide Bass Amp Designer:
Once the installation is finished, you no longer need the installer file open. GarageBand 10.0.3 dmg
Troubleshooting
Users who previously "purchased" GarageBand (when it was a paid app or tied to an Apple ID) can often find it in their Purchased tab in the App Store to download the last compatible version for their system. GarageBand 10.0.3 dmg
A: Delete the app from Applications and empty Trash. Then delete ~/Library/Containers/com.apple.garageband10/ and ~/Music/Audio Music Apps/ . GarageBand 10.0.3 dmg