Toilet flange concreted into floor, replacement?
You have several possibilities. My recommendation would be to cut the flange and install a new flange in side the cast pipe something like an oatey  43539  20$.

By cutting the old flange out this will allow the toilet at the same level as the previous one. 

there are some thin metal repair flanges that fit over the existing cast but my experience with these is they do not last and cost 15$ the cost difference 5$. 

I have also used a wire welder to repair a flange in the past where the flange was installed in the middle of a bend (directly welded no straight pipe at all) my option was to weld it up or cut the slab to repair and after grinding to clean metal I was able to weld it up sliding a piece of thick copper under it when done the copper slid out and I had a space to drill and tap new holes , so far this has held up for + 5 years. 

There is always the least desirable option to cut the slab and replace using a fernco or “no hub” and change to plastic but the set and twist from oatey would be my option.