Replacing a toilet seat is one of the simplest home repairs. No plumbing, just a screwdriver and pliers, and the whole job takes about 15 minutes.
Pry up the plastic caps at the back of the seat. Hold the nut under the bowl with pliers and unscrew the bolt from the top. If bolts are rusted, apply penetrating oil first.
Wipe the porcelain around the bolt holes. Line up the new seat, drop the bolts through, and thread the nuts from underneath. Tighten snugly — do NOT overtighten against porcelain.
Sit on it and wiggle side to side. The seat should not move. If there's any shifting, tighten each bolt another quarter turn.