how to replace a toilet flush seal?

You give a slightly unclear problem description.

The typical leak problems are -

Cistern overfilling and consequent overflow dribble into the WC pan. Usually fixed by adjusting water level.

Defective seal between pan and close-coupled cistern with consequent dribble onto floor at flushing time. Usually fixed by tightening or replacing doughnut washer.

You seem to be blaming the seal between the syphon and the bottom of the cistern - are you sure of this? Where is the dribble running to?

Peccavi
July 2010