1. Check the pressure on you boiler (normal pressure is between 1-2 bar), if the pressure drops to 0 will show F2 fault code.
2. If the pressure is below 1.5 bar, you can top up by using the filling loop, in this case is an internal filling loop with black leavers. You have to open the black leaver valves, one by one. I suggest to keep your hand on the second leaver after you opened, because sometimes is just take a few seconds to fill up till 1.5 bar and then you can close strait away.
3. After the pressure reached 1.5 bar you have to close booth black leaver valves, one by one. In case if you didn't managed to close on time and the pressure is above 2 bar don't worry, you can just bleed a radiator and then the pressure will drop automatic. If the pressure is above 3 bar, then the boiler Pressure Relief Valve (PRV) will open and will discharge the water trough PRV pipe and the pressure will drop automatic.