How can I replace a plastic toilet flange that is cemented into a slab floor? It appears to be attached to black ABS piping.

there are a number of ways to replace the flange on a waste line coming up through the floor. Everything will depend on the size of the opening. If the opening is 4" everything is great. You can simply break off the old flange or cut it off, and glue a new flange on the inside. You have to buy a 4 x 3 flush fit flange and it will glue on the inside of a waste pipe. You probably have to buy this through a large warehouse store or a commercial plumbing store. If the pipe is 3" then you have two solutions. The first is to break up the floor around the flange, after removing the collar and chisel off the old flange very carefully, then glue on the new flange on the outside of the pipe and recement the floor. The second is to buy (a specialty flange) that is 2-1/2 inches in diameter and fits on the inside of the pipe. You can only get them through plumbing wholesalers. In most states thay are not legal for initial installations, but some inspectors allow them for retro fits.

You can also Replace the flange alone with a replacement flange kit. This requires a great deal of skill to get it right. All flanges then must be bolted to the floor so that they don't pull up when you tighten the toilet to the floor. This is very important, because most people overtighten the bolts on a toilet and break the flange. Make sure you buy a stainless steel flanged closet collar as they will not rust and corrode.

Of course you can always call a professional too.
Good Luck

Raymond VinZant Plumbing Prof.