Select the audio region you want to reverse by clicking on it in the Garageband mix window.
Open the edit menu. Choose "Add To Loop Library." Enter a unique name for the region so you can easily find it in your loops folder. Click on "Create."
Open the folder "Library> Audio> Apple Loops> User Loops> Single Files". In that folder is an .aif file of the audio you selected to export for reverse processing. Select that file and choose "open with> Audacity". The file will open in the Audacity audio editor window.
Press "Apple A" to select the entire file (in Audacity). In the Effect menu, choose "reverse." Press the space bar to play back the reversed audio file. In the File menu, choose "Export as .wav file," and save the file with a new name onto your desktop.
Switch back to the Garageband application and simply drag and drop the reversed file onto the mix window. The file will now appear on a new track, ready for playback.
Align the imported reversed audio file with your original file. Play back the audio and adjust the start point of the reversed audio file as needed to fit into your musical arrangement.