First day user here....

I'm thoroughly confused when I go the change the speed of a track. I have specific BPMs that I would like an entire track to be. Let's say song is currently playing at 113 and I need it to be slowed down to 90. So when I open up the window to change the bpm to 90, then play the track it's sped up ridiculously fast verses slowed down like I wanted it to be. Please explain to me why this is? I was always taught that the less beats per minute means the slower the music. It seems to be the opposite in Acid Pro 7 and I'd like to know why, and what I can do to change it, or understand this software better so if I want a speed of 90, it will give me the speed of 90 regardless what numbers are in the bpm....help!!!!!!