Replacing a stuck-on seat.
The hardest part of replacing a toilet seat is removing the bolts that hold the old seat in place. These bolts are fastened by nuts beneath the toilet bowl. Use an adjustable wrench or a socket wrench to unscrew them.
If the bolts are metal and have corroded, drench them with penetrating oil and let them soak overnight. If the bolts still resist, saw through them: raise the toilet lid and seat, mask the rim of the bowl with cardboard to avoid scratching it, and saw through each bolt with a hacksaw.
Before installing a new seat, clean around the bolt holes. Coat the bolts of the new seat with petroleum jelly to facilitate future removal.