You can get a bottle of stuff to do it, Dr Beekman i think, but I do mine with thick toilet bleach. I wipe around the rubber where the water sits, pour some through the soap and softener dispenser and put the rest in the drum then run it empty on a long hot cycle. I then wash something in it that won't matter if it gets bleached like dusters for a couple of washes.
As Woofgang says you should run a 95 degree cycle every once in a while. You can add a little bleach too but be carefull not to add too much as some can foam up quite abit. I also remove the soap dispenser and give it a good going over withsome hot water and an old tooth brush. Keep the door ajar when not in use.