jeff beat me to it. remove the nut that holds the prop, remove the prop (some bits (of the old shear pin) may drop out) there will be a pin through the shaft which you will probably have to punch out with a drift.
get a new shear pin from chandlers, these vary in hardness/softness (as well as size) get a selection, use the softest that will allow the engine to run normal operation without breaking.
vic, October 2012