This is a good question, in short, it`s possible but not straightforward, especially for warm, high fill weight down sleeping bags. 

Having a large enough volume place to wash (hand wash is best) is the first concern, most people use a bath. Then you need to be able to rinse well and dry, without damaging the down.

In reality, unless it`s a superlight down sleeping bag with minimal filling you can handle easily when wet and dry effectively in a tumbler then we would recommend a specialist cleaning service from an outdoor gear service / cleaning company.

Down bags are mostly a serious investment and with the right care can provide years of high performance service. So paying for a professional clean is definitely worth it.

For this we`d recommend the companies below, contact them to get more details

Scottish Mountain Gear https://www.scottishmountaingear.com/

If you have a Rab sleeping bag then they offer a direct service for their products only, https://rab.equipment/uk/rab-wash