It says on the tags that you can use some mild soap (maybe some gentle dish soap or Dove?) and warm water to spot-clean him and then let him air dry. After he dries, brush the fur to fluff it up again.
That said, if he just needs a good overall bath, I've heard that putting stuffed animals in pillowcases and tossing the whole thing in the washer works. Not sure if that would work for Giant Microbes, though, and I wouldn't put him in the dryer.