replacing tiolet mechanism?

I take it you mean the syphon flushing unit.
Close coupled toilets are a pain you have to take the cistern off the toilet pan to be able to remove the whole syphon unit as the securing nut is under the cistern. Turn off the water disconnect the water feed pipe from the cistern. Bale out the water or flush it if it can be done.
Unscrew the wall securing screws,.
Undo the cistern to pan securing bolts.
lift the cistern off the pan tip out surplus water if any,
turn the cistern over undo the syphone securing nut remove the syphone. Reverse installaton. Also while off replace the large seal between cistern and pan.

Nobby
April 2009