Wrap the teeth of tongue-and-groove pliers in a soft cloth. Place the teeth of the pliers around the aerator on the bathtub spout.
Turn the aerator counterclockwise to remove it from the spout. Check for an O-ring or washer inside the aerator and carefully remove it.
Take the aerator to a plumbing supply store or home improvement warehouse to find a replacement. Aerators have either male or female threads and come in slightly different sizes. Ensure you get an aerator that matches the original.
Install the washer or O-ring onto the aerator. Hand-twist the aerator clockwise, then tighten it with the cloth wrapped tongue-and-groove pliers.