It’s easy, all you need is bread and patience.
Just stick a piece of bread in the bag of hard sugar and close it tightly. The brown sugar will absorb the moisture from the bread leaving it soft after 24 hours. You can also use a terracotta coin, but you must remember to soak it in water first. Or, a slice of apple will work too, but I find that it leaves an apple odor in the bag.
To me, a piece of bread is your best option for softening rock-hard brown sugar.