It would help to know the nature of your struggle - you do not say.
Hold the blade still and turn the central nut/screw anticlockwise - if necessary have somebody bash the spanner while you hold it firmly on the screw head - to help it begin moving round.
You might consider a squirt of penetrating oil like Plusgas £6 post free from Amazon.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lawn mowers.