When you look at the toilet seat hinge there should be two "tabs" attached to the ends of the hinge. Directly below the tabs on the hinge are the nuts that need to be unscrewed. Reach under and you should be able to find them with out looking under. If the seat is not to many years old the nuts will be plastic and should easily unscrew. If the whole bolt is turning and the nut will not come off you will need to pop the covers off of the two tabs at the ends of the hinges. More often than not the covers will need to be flipped open from the front (the seat side) toward the back (the tank side).

Now having said all of that if you have a really old toilet none of the above will apply. My recommendation would be to call Blast and after he is though with the toilet call a contractor and have the remains of that end of the house remodeled. HTH

What OBG said also. I know he has changed more than i have, i just sold seats always tried to avoid the installation part of it.