Denying offer of admission from graduate school after accepting
Presumably the school you heard from was not your first choice. 

I'd just be as upfront as possible, with everyone. Next step is probably to contact the schools and/or professors you haven't heard from, say that you have another offer, and ask if they can give an update on the timing of their decisions. 

Depending on what those schools say, contact the school you were accepted to and request another extension until the specific dates that the other schools gave you.