How to Replace a Cast Iron Toilet Flange
Seal the open to the sewer line with an expansion plug or large towel to prevent debris from entering the drain system.
Drill out the old lead seal that is holding the cast iron toilet flange in place. This is the soft metal seal that is in between the outer closet flange and the inner sewer pipe. Once you have have drilled as much lead as possible, remove the remaining lead with a heavy screw driver.
Remove the Oakum packing material that is located under the lead seal with a heavy screw driver or chisel.
Remove the cast iron toilet flange, you may need to use a hammer and chisel to loosen or even break the remaining flange.
Install the new mechanical closet flange in place. Level as evenly as possible. Tighten bolts to the manufacturer's specifications.