I already paid for my federal return and opted to mail it in. I just realized I left off a W2 for Sick Leave pay. how do i go back and change my federal return.

I have a W2 that is for Sick Leave pay only. It has no income in box 1. I just realized I didn't receive that W2. How can I change my federal return that has already been paid for? I opted to mail it in so it hasn't actually been sent. But I have already paid.