@gailcalled has it right. Wash it on gentle with a mild detergent and rinse it. I have done this and it works.
I have also dried it without the use of tennis balls, but I’ve heard of this technique. You want the lowest temperature setting on the dryer.
One question: is this a box stitch design or some other? In other words, are the down feathers divided into separate boxes in the quilt? If so, this is good. It will reduce shifting of the feathers during washing. If not, then be prepared for the feathers to work themselves to the edges of the quilt.
Edit to add: use a duvet cover to reduce the necessity of washing the quilt itself.