Open your GarageBand project and determine if you want to preserve it as one single recording or if you want to keep all of your individual tracks in tact. Your "tracks" are the horizontal spaces on your screen that contain the individual layers of your song, such as the guitars, the bass and the vocals, for instance.
Export your song project in its current state. If you have only one track or if you want to export your tracks as one single audio project, just click "Share" on your menu bar and then click "Send Song to iTunes." If you want to preserve your individual tracks, click the "Solo" button on your first track and then click the "Share" menu followed by "Send Song to iTunes." Do this for each track, one at a time. The "Solo" button appears on the left side of the track and looks like a pair of headphones.
Highlight the track regions with your mouse and click "Delete" to remove them from the project. The regions are the shaded, colored spaces on the tracks, which indicate your recordings.
Click the "Tempo" indicator at the bottom of your screen. A small vertical bar will then appear. Drag the slider upward along the bar to speed up your tempo, or drag it downward to slow down the tempo. If your "Tempo" indicator doesn't appear in the blue LED display, click the arrows on the left side of the display until you see the word "Tempo."
Click the "Media Browser" icon at the bottom of your project window to see a list of audio files in your iTunes library. Click the track titles that you previously exported to re-import them into your project, completely unaffected by the tempo change.