If you are using a replacement disposer that is similar to your existing model you may be able to skip this step. Compare the supplied mounting assembly with the existing assembly. If they are the same, it is not necessary to replace the assembly and you can skip to the next step. However, if the visible part of the flange in the sink is corroded, leaks or you would prefer the shiny new flange, then proceed with this step.
Loosen the mounting assembly by turning the three screws enough so that the assembly hangs loosely. Remove the mounting assembly from the sink flange by prying off the snap ring. The snap ring sits in an indentation in the mounting assembly and has a small gap. Pry the ring off at the gap with a screwdriver. The assembly should now drop off of the flange.
Lift the flange out of the sink. Scrape away the old sealant material and clean the surface before replacing the flange. Also remove any gasket or sealant from the underside of the sink.
Roll about 4 ounces of non-hardening plumber's putty into a rope about 3/4 inch in diameter. Apply the putty around the indentation in the sink to make a complete seal under the flange. Place the new flange over the opening and press it firmly in place. Trim away any excess putty with a screwdriver or putty knife.