how to randomly change a probability?

I have a set of items, each of which is associated with probability and I would like to randomly modify this probability.

I want the ability to control how much the probability is randomly modified by. For some probabilities i would like to modify them by a small percentage and for others I would like to change them by a large percentage.

I can only imagine swapping probabilities between different items.

I have considered to use java.Util.Random but I dont see any methods I can use.

Thanks.