how can i fix my caroma concorde cistern?

how to remove worn seating washer from my caroma concorde dual flush cistern .after removing outlet valve assembly to get at the seating washer, does the nylon retaining washer unscrew or is it pressed on? dont want to force and break something! my unhelpful "fix-a-loo" bubble pack instuctoin is very brief, ie turn off water,flush,replace seating washer,turn on water.