Usually once you have disembarked the aircraft, walked up the air bridge and then gone down the stairs to the lower level of the terminal, you turn left to take you to Baggage Reclaim/Exit. Turn right and follow the corridor along where you come to a set of stairs which takes you back up to the gates at the top of the air bridge/departure lounge. No need to exit and re-clear security.

Be aware however that very occasionally they stick a man in a high-vis in the corridor to prevent you turning right to go back up to the departure lounge. I saw this last week after arriving from LHR.

MAN-FF