Its done via jumpers on a multipin plug inside the amplifier, (think it has 9 or 12 pins, its been a while) you need to find the appropriate section in service / user manual and change the jumper cables in the plastic plug housing. check check double check and check again before re-powering amp.
Ok, thanks for the reply. Is the user manual online? All I can find is what looks like a 20 page brochure and the complete electrical diagram of the entire amplifier. I looked at those, but it was very confusing. There was to many pages, and I didn't now where to look.