Check the contract you signed with the attorney in case there is a specific procedure for firing him that you agreed to. Otherwise, simply send the attorney a letter stating that you hereby terminate the representation.
I agree with the prior answer. You should also review your fee agreement to be certain how much, if any, money would be returned to you.
If your case is in mediation, I suggest finding another attorney pronto. Be clear with any new attorneys you interview why you fired the last one. There are lawyers that won't touch a case in which the last attorney was fired.