replacing. If you are a diy'er you could attempt the repair yourself but it will require shutting off the water supply valve for the toilet., then flushing all of the water out of the tank. disconnecting the water supply line from the tank or the valve, either end will work. Now you will have to loosen the two nuts at the toilet base that hold the toilet to the flange coming out of the floor.once the two nuts are removed, and sometimes they can be stubborn if the toilet has been installed for a while. lift the toilet straight up and set down on a stack of newspaper. sometimes the old wax ring may stay with the toilet, if so remove it. If the wax ring stays on the flange ,,remove it and instlal a new one on the flange...then repeat the procedures to re-install the toilet.
go to this URL website for pictures and step by step instructions for changeout.
all else fails call a plumber to changeout for you. good LUCK LJ