I wouldn't even mess with this thing. You need to replace the string, too. A string should be changed every three years at the most.

To answer the other questions I'll start from scratch. You have a string and cable on your bow. New or used doesn't matter. If you want to increase the drawlength you twist the cable shorter or untwist the string (or both). This will also cause a corresponding weight increase.

If you want to decrease the drawlength then you twist up the string (shorten it) or untwist the cable (or both). This will cause a loss in peak weight along with the drawlength change.

If you want to maintain the present draw but gain peak weight then you twist up both the string and the cable, putting more preload on the limbs. This also causes the ATA to get shorter and the brace height to become taller.

One thing I would say is that you also need to take a pencil and mark the side of your cam where it passes through the limb. This will give you a visual check of which way the cam is rotating when you start twisting things up or down. This is one of the first things I do after I shoot a new bow about 100 shots to settle the string.

Something else you can do with the pencil marks is, over time tell whether your string is stretching out and how much. If you got factory string then you will most likely be adjusting them quite often. Get some decent aftermarket strings.