If you're not careful, brown sugar clumps up and gets hard pretty quickly. The Kitchns suggest that the easiest remedy for this is a microwave, a few seconds, and a wet paper towel.
All you need to do is put the brown sugar in a bowl, cover it with a wet paper towel then a plate, and microwave for about 20 seconds. It's similar to a tip we've shared before with water, but it's a bit faster.
The Best Way to Soften Brown Sugar in a Hurry [The Kitchn]