How do I restore the local save backup file for World War Z: Aftermath?

If your in-game progression was deleted, you can try to restore it by using your local save backup.

This method won't work if the data was lost after you have reinstalled your OS.

Go to %LOCALAPPDATA%\Saber\WWZ\client\storage\ folder.
You should see the slots folder, which consists of your backup save files.
Copy user_profile.dat, user_progression.dat, user_firearms.dat, user_challenges.dat, user_loadouts.dat (each file contains respective data) from one of the folders in slots to ...storage*profile* where *profile* consists of random letters and numbers (like 8f6c79c53f204e38bd...).
When launching the game, choose Local Data if the launcher asks you which data to use.