You cannot without paying again, afaik.
Do you have a TimeMachine backup of your system? Then try to restore the following folder in your System Library from your backup:
For example, to restore sting's Roxanne:
/Library/Application Support/GarageBand/Learn to Play/Artist Lessons/Sting_Roxanne.mwand
I just discovered, you should be able to download the artist lessons again from the App Store, see this Support Article: