Disconnect the plumbing from the tailpipe extending from the flange. If it's connected to a P-trap (the curved pipe under the sink), loosen the lock nut that holds the tailpipe. If the plumbing is PVC plastic, use your hand to loosen the fittings. If it's metal, use an adjustable wrench or channel-lock pliers. If the disposal is connected to the flange, turn off the power to the unit before disconnecting the plumbing. Loosen the lock nut that holds the disposal drain in place and disconnect the pipe. Release the disposal from its quick-lock connector and lower it to the base of the cabinet.
Remove the nut holding the flange, using your hands or channel-lock pliers. If it cannot be loosened, squirt spray lubricant on the threads and let it sit a few minutes before trying again.
Push the flange up into the sink basin. Clean up the old putty remaining in the flange hole, and wipe the surface clean with a rag.
Roll plumber's putty on the counter with your fingers to make a ¼-inch rope. Circle the drain hole with the putty rope, ensuring that there are no gaps. The putty serves as a seal between the flange and sink.
Push the new flange into the hole and flatten out the putty. Use the manufacturer's washer and nut to secure the flange in place. Tighten it by hand until it lies flush with the sink surface. Use channel-lock pliers to tighten the last turns.
Clean up the putty that squeezed out during tightening, using a rag or towel.
Reconnect the plumbing you disconnected. Run the water and check for leaks, both in the piping and around the flange.