A toilet seat that shifts usually has loose mounting nuts, worn washers, or a hinge that no longer holds alignment. Tightening should be controlled so the ceramic is not cracked.
Inspect the fasteners
Lift the hinge covers and identify the bolt and nut arrangement. Some seats use top-tightening anchors, while others require access beneath the bowl.
Center the seat first
Align the seat evenly with the bowl. Hold it in position while tightening each side a little at a time so it does not twist.
Use moderate pressure
Tighten until the seat is stable, then stop. Overtightening plastic hardware can strip it, and excessive force can damage the porcelain.
Replace worn hardware
If the seat loosens again, replace the washers or mounting kit. A cracked hinge or damaged bowl should be assessed before continued use.