Remove the Disposal Unit
Support the bottom of the disposal with one hand, or have a helper support it. Insert a screwdriver shaft into one of the ears on the lower mounting ring of the disposal, and turn it 1/4 turn counterclockwise to release the disposal from the mounting assembly. The disposal will come straight down so be ready to catch it.
What is left is the mounting assembly. To remove it, loosen and remove the screws between the upper and lower mounting rings. Take off the lower ring.
Use a flat screwdriver to pop the snap ring off of the cylinder of the sink flange that fits down through the sink hole. Slide the triangular backup flange from the sink flange, followed by the fiber gasket. Push up on the sink flange to break its seal with the sink, then lift the flange out from the top side of the sink.
Clean around the sink opening with a rag to remove gunk and old plumber's putty.