How do you undo the top nut am I supposed to turn the top section or the nut underneath. Either way its just spinning around. Thanks.

Grip the top with something (grips/adjustable spanner) and undo the nut. If it is really stuck put the top of it in a vice.

as paul says the centre shaft has a flat on it to stop it from spinning to undo the nut. chances are it will be too tight to hold that with a spanner so mole grips are prolly your best option. obviously you need to clamp the springs beforehand. last time i changed springs that had been installed for sometime, i used an air impact wrench to shock the nut lose, then wound it the rest of the way with moleys.

yeah, as above. a good powerful airgun does it every-time...

Locked mine in a vice and undid the nut that way.