You remove the High Tension (voltage) cap on the top of it - fit a Spark Plug Spanner - and unscrew it anti-clockwise.
Then you take a plug of the same type and work backwards through the process.
If the plug you take out is dirty with black soot you might think about cleaning it up - dry toothbrush - fine sandpaper -and resetting the gap.
http://www.youtube.com/watch?v=Iobn0fCun9k
Peccavi, November 2012
