Uh...yeah.  Change the tempo of the arrangement.  It's difficult to see on the website - it appears to be labeled BPM to the right of the time clock in MM-MX.  But, it's only going to work if you're using soundpool objects or files with a tempo assigned to them.  Otherwise, some files will need to be time-shifted manually.

If you're making drastic changes, you're going to get some distortion.