Hello, It is fairly easy but requires a bit of patience. The cones are held in place by a split pin (sometimes called a sergeant pin) that needs driving out by a nail or similar piece of metal the same diameter as the pin, BUT, before you do this you need to put a securing pin or clip (like a giant kirby grip) through the shaft with the big spring on to hold it in place and stop it unwinding. A hole to do this is near the cone. If you secure the main shaft with such a pin, you can drive out the cone securing pin which will let the cone slide off the shaft and you can replace it with the new one. The critical bit is to get the tension the same in both the left and right wires. Turn the new cone with the wire attached until it appears to be at the same tension as the existing one (ping it like a guitar string - get the same note). Take out the securing pin and you're done! A mate and a big screwdriver with the same diameter of the securing pin is useful as the securing pin of the cone may not line up with the direction that you can drive out easily.
Hope this helps. Cheers, Andy