you will follow the signs for connecting passengers and go through immigration and customs at the C concourse, once through security you will go upstairs and there is a departure screen right there to see where you have to go, most likely you will have to take the new train to the A concourse if your flying united express.

If your flight to IAD from EWR leaves on time you should have plenty of time, worst case scenario your plane is sent to the A concourse then you just take the train over to C concourse.