VHHH has 3 runways, but in P2ATC, it's still showing the old runway, I am in the latest AIRAC, how's P2ATC get the airport runway config from? I play around the data maintenance but I can't find a way to modify the runway, like the old 25R will be 25C now.

The runway data used for flight planning and vectoring is taken from AIRAC / Navigraph data, so if it's in there, the new runway should show up for flying.

DataMnt Runway data is, unfortunately, not available in Navigraph.  This data is in a separate file and, depending on what SIM you have, can be imported from the SIM, if it has the new runway.  But that might be some time after it is opened before the SIM picks it up.  

So, if your DataMnt data doesn't have it, you won't be able to get taxi instructions to/from the new runway.  You can just taxi to it manually, looking at the satellite map, assuming it has the new runway on it.

Dave