Krusty is correct. But when you know this you have not yet completely solv ed the problem. The blade has to be rock solid to undo the nylon nut. To do this jam a short beefy piece of timber (say 70 mm square x about 18 cm l ong between the end of the blade and the inside of the plastic frame of the mower to prevent the blade turning anti-clockwise. Then use an adjustable spanner to to turn the nut anti-clockwise. Job done.
AJKM Somewhere in France