https://puredown.com/blogs/blog/how-to-care-for-a-feather-bed-mattress-topper
You can do it yourself, there are plenty of online guides but in general use the extra large capacity front loader at the laundromat, delicate cycle and don’t over soap or use harsh laundry soap - something like Woolite or other delicates soap would be best.
If you decide to tumble dry on low in the large capacity dryer, bring a set or two of wool dryer balls to put in with the topper that will help speed the drying process by fluffing the topper as it tumbles.