Make and model would help if its thetford go to their website www.thetford-europe.com (where ever you are) fill in the online contact form and ask them. You can also download pdf its one covering all models you open then search for your model.
Remember next time make and model as yo"ur question is like 'how do I change clutch on my car' no make or model, methods differ.
Bob uk, March 2016
