You don't need to make an appointment. Just show up and wait. It can be quite hectic for where to get a number and stand in line, but just poke around and most likely some agent will point the way.
By the way, some people recommend to not do the renewals (at migraciones or border crossings) and to just pay the fine on the way out. Besides possibly being marked in their system somehow for violating some sort of statue, and risking the fee going up before leaving, what happens in the meantime if a person is stopped in the street and asked for the ID or you get in an accident, etc and they see you've overstayed?