Tools: Philips, flat head screwdriver and the bucket.
Turn off the main gas shut-off valve that supplies water heater.
Turn off the main water shut-off valve that supplies the unit. It is usually located close to the water heater.
Open the hot water tap(the lowest located) to relieve the pressure from the unit and drain the unit.
Locate the water valve, put the bucket under, remove the retaining clip and the bushing.
Once the bushing has been removed, there will be some water coming out from the heater and the water lines.
Locate the hydro generator and disconnect the wire connector.
Remove both clips from the hydro generator, disconnect and separate the water pipes.
The hydro generator is now removed and the new one should be put back in reverse order.
Make sure that the piping, clips and the inlet filter are properly installed.
Open the main water shut off valve to check for possible leaks.
If there are no leaks, open the gas supply and resume the water heating operation by turning the unit ON.
Detailed guide here.
(photo credit to Bosch)