To soften brown sugar, you can use a microwave oven. Start by putting the hard sugar cubes in an oven-safe container.
Take the container and place it into the hot spot of your microwave for about 2 minutes, or until moistened. It will still be crumbly but should be softer than before.
Alternatively, you can also place them in an airtight container on top of the stove with an inch of water underneath, on medium heat for 5 to 10 minutes or until they are soft enough to go through a food processor or crumble easily by hand.
So this is how you can soften the brown sugar.