On this page of our game guide to Batman: Arkham Knight we provide a description that explains how to get to the computer in order to regain control of Mercy Bridge. It's located at Grand Avenue Station.
Head to Grand Avenue Station located in the south-eastern part of Miagani Island. You cannot use the Batmobile, so you should rely on gliding to keep away from the streets (there are a lot of enemy units there). After reaching your destination, get to one of the vantage points and scan the area using the detective mode. You will learn that the computer which controls the bridge is guarded by eight armed thugs.
As usual, try to eliminate the enemies silently. One of the first opponents that you should deal with is the drone operator standing on one of the balconies. When you do that, you will also disable the flying machine. Attack the rest of the enemies in any order and use the objects from the environment (vantage points, ventilation shafts, weak walls, etc.)
When the fights are over, search for the terminal and hack it. Listen to a conversation with Alfred and watch a recording prepared by Riddler.