With an AIO, such maintenance is not really provided for. Hence the reference to the RMA. Otherwise, the only solution is to remove the AIO completely, empty it via the fill port and refill it via the fill port.
The procedure is similar here. Fill with distilled water, let it run, drain everything again and then refill.
If the cooling fins are clogged, you can dismantle and clean the radiator bottom after draining. To do this, loosen the 4 screws at the bottom and remove the bottom. But since it's an AIO, you would lose the warranty if you didn't discuss the whole thing directly with the support team. That's why we don't have any instructions for this, because it's not really what customers want to do.