You have saved your project somewhere - likely in ~/Music/GarageBand. Just find the project file (in the Finder, not in GB) and rename it just as you would any other file - click twice, not too close together, on the title in the Finder.
You have saved your project somewhere - likely in ~/Music/GarageBand. Just find the project file (in the Finder, not in GB) and rename it just as you would any other file - click twice, not too close together, on the title in the Finder.