go to the mixer....hit the MODE button and then Pad 8 I think. It should have mixer written right above the pad. Once you're in that screen press the pad with the sample that you want to change the volume of. Then use the jog dial to lower it accordingly.
Next time read the manual first

"There's a lotta bull shit that's flooding the scene, where everybody's a star, and hot shit is few and far between." -Black Thought