Just loosen the two grubscrews in the aluminium hub of the lowest gear, and the shaft - including the upper rotor head - will lift out.

The blade holders on the rotor head dismantle by unscrewing the two screws on top and slide apart.

Be aware of the two flats at the bottom of the shaft. Ensure that the grubscrews of the gear hub align with and tighten onto these when replacing the shaft.

Also be aware that there is a ball bearing in the hub of the upper - outer shaft - gear. It's possible for this to fall out when the lower gear is removed. There may also be a washer between the two gears.

You will probably have to remove the battery holder and, maybe, the landing gear to gain easier access to the gears.