I take it you mean syphone unit this MAY mean removing the complet old syphone from the tank which means removing the cistern if a close coupled to pan. If its a separate one that has flush feed pipe (pipe diameter 1.5" or 32mmx 300mm long approx) from cistern to the rear of the toilet pan then cistern tank can stay in place remove the pipe and unscrew the collar nut under cistern to remove the pipe then undo the other collar nut holding the syphon in place . MAKE SURE YOU DRAIN ALL WATER FROM CISTERN FIRST.
Talk to local plumbing store on the one you have take a digital photo of it to show them.
