If you don't have access to a repair center. You will have to take the phone apart, using the ifixit guide, to the boards and clean with isopropyl alcohol 97%. Then use a ultrasonic cleaning machine. You can use a toothbrush to get corrosion off the board, but brush gently so you don't damage any components. Here is a link to an ifixit video on repairing water damage. Water Damage Repair video Good luck