Pretty simple. Instructions from Holden Service Manual below. Suggest you also replace the thermostat at the same time. I always use a thin smear of blue sealant as well as the gasket just to make sure there are no leaks.
REMOVE
1. Remove engine trim covers. Both inlet manifold trim covers need to be removed before the front cover can be removed.
2. Disconnect radiator upper hose from thermostat housing and tie up so that the hose end is higher than the engine to minimise coolant loss.
3. Remove the two bolts securing thermostat housing outlet cover to inlet manifold.
4. Remove thermostat housing outlet cover, 'O' ring and thermostat.
REINSTALL
1. Clean mating surfaces of inlet manifold, thermostat and thermostat housing.
2. Reinstall thermostat in inlet manifold with 'TO RAD' inscription positioned towards radiator.
3. Install new 'O' ring into housing cover.
4. Reinstall thermostat housing outlet cover and progressively tighten bolts to specified torque 20 - 27 Nm
5. Reconnect radiator upper hose to thermostat housing.
6. Reinstall engine trim covers and tighten fasteners to the specified torque. 6 – 14Nm. Note that the inlet manifold covers are specific to each side. Reinstall with the correct orientation.
7. Refill cooling system with specified coolant and check for leaks.
