Place the hard and lumpy brown sugar in a glass, microwave-safe, container. I used a glass measuring cup.
Next, dampen a paper towel with water. I wet my towel and then squeezed it out. You want it damp but not soggy.
Place the dampened paper towel directly on top of the brown sugar.
Cover your bowl with a lid or plate that is microwave-safe.
Place your covered glass container in the microwave on high for 20 seconds.
Remove from microwave and stir with a fork breaking up the brown sugar chunks. This will not be hard to do as they are now soft chunks, not hard.
If necessary, repeat the cycle until all of the large, hard chunks are gone. I had to do this 3 times for my bag of brown sugar but it worked great and was so worth it.