Its been a while since it happened to me, but I got a medical, thought I had looked it over carefully, signed it, but when I got home realized that they screwed up my address when typing it up.
I forget exactly how I went about fixing it, but I'm pretty sure I either called or emailed them and they sorted it out really quickly. They even gave me a call a little less than a week later to follow up as to whether the new certificate had arrived. I was very impressed with the level of competency and customer service.