You can slow down the song, but not individual tracks of a song arrangement.

To slow down your song open the Tempo Tracks from the Track menu or use the key combination ⇧⌘H

Then drag the control points in the Tempo track up and down to change the tempo for selected measures. All software instrument tracks and audio regions in audio tracks with flex enabled "follow tempo and pitch" will change to the new tempo.

If you want to change the global tempo and not only selected bars, you can set a different tempo for the song in the LED display. Set the LED display mode to show the Project, not the time. Then click the tempo to change it.