There are a of couple options! You can use your hands to push the down around or a tennis racket to smack the down.
Pro tip! You can also toss your bag in the dryer on no-heat (air fluff) and add four clean tennis balls, the static created by the tennis balls will help separate the clumps of down. Note, this may take a few cycles. Don't panic if nothing has happened in a
20 minute cycle, just take the bag out, hand-fluff, and try again in the dryer with no heat.
Unfortunately even following these instructions carefully can cause down migration and internal damage to the seams of your sleeping bag. We recommend spot cleaning your bag before risking damage by fully washing the bag. Due to the risk involved in washing your sleeping bag, damage caused in that process is not covered under our warranty.”
Do NOT put your sleeping bag in the wash if you can avoid it.