You also need to pay a premium on the annual membership, but it's not prohibitive. Becoming a USAS coach is a bit more involved. Check your USAS LSC web site to get started.
Sounds like you are asking how to update your membership. Go to your My USMS account and then click on the Membership Information. There is a place there to say you are a coach.
If you are already a certified coach, yet looking to add the designation on mid year so your coaching certification is present on your membership, you can do so through this link - https://www.usms.org/reg/member/updateinfo.php - please feel free to reach out to membership@usmastersswimming.org if you have any other questions!
