Dosset's answer is correct if you are of a practical nature. Briefly, you need to cut off the water supply to the cistern. Flush (if possible) then split the cistern from the pan (assuming it's a close-coupled toilet) by undoing the two nuts (often wing nuts) under the cistern. This should allow the cistern to be removed from the pan (there may also be a couple of screws holding cistern to the wall). The syphon mechanism can now be released by undoing the large nut at the bottom of the cistern and by uncoupling the link from the handle to the lift paddle. You can then check the condition of the water lift diaphragm and replace as necessary. Diaphragms and complete syphon mechanisms are readily available if you have a B&Q or plumber merchant nearby. Hope this helps.