"Split" that part from the rest of the track by placing the cursor at the beginning of the section you want to lower and hitting the "S" key, and then doing the same thing at the end of that part. Then you place your mouse at the top of the part you want to lower and move it downwards. It will tell you how many db's under 0db you're moving it.
If it's a "pop" that you want to get rid of, then you might be better off zeroing in on that part, doing the same splits that I mentioned, but then instead of lowering it, delete it completely.