If the audio isn't already 'looped' to automatically follow tempo changes, do this:
- Ctrl+A to Select All.
- Open the Clip tab of the Track Inspector, and expand the Audiosnap section.
- Click to check the 'Enable' box, and give it a moment to finish processing.
- Ctrl+A again, and check the box for 'Follow Project Tempo'.
- Change the initial tempo by one of the methods John mentioned above.
Depending on what all is in the project, you will then want to open the Audiosnap palette, select individual clips/tracks and set an 'Offline' rendering algorithm appropriate to the material, and then Bounce to Clip to render the stretching permanent with the Offline algorithm.
Of course you'll want to do all of this in a copy of the project because the sound quality is never going to be as good as the un-'stretched' version (actually
compressed in the case of increasing tempo, but 'stretching' is the generic term for changing the length of audio), and you can't easily reverse the changes once they're rendered and saved.