Assuming close-coupled cistern (mounted on top of the bowl) Turn off water - empty cistern - disconnect water supply pipe, loosen and remove two nuts under cistern which hold the cistern/bown together. Remove 2 screws fixing cistern to wall and lift off the cistern.
There should be a big plastic nut under the base of the cistern, probably with a rubber 'doughnut' covering it - remove it to release the syphon. Also release the lever connecting the flush handle to the syphon. Obtain a replacement syphon, clean the cistern surfaces where the sealing washers sit, reassamble with a dab of jointing compound, and put everything back together.
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