cntr+shift at the end of a clip then drag adjust to your taste
(important)
do bounce to clip after you are done(this will render the clip to a very good quality

AudioSnap or slip stretch. The former provides the ability to set stretch percentage and algorithm in the Clip and Track property inspectors. Bounce the clip(s) to apply destructively. Another destructive method for stretching clips in Platinum is the Time/Pitch Stretch 2 DX effect in the clip context menu under Process Effect > Audio Effects.