In the Welcome screen, click Quick setup and Enter firm details toward the bottom is where you enter your EFIN. Correct it now. 

Personally, I don't think there is much you can do about efiling one return with an incorrect EFIN. If it was accepted the client return will likely be processed.

YOU may be a letter later, but filing superseded return is not very easy and since the change does not affect the tax, I would not bother.