There is a large opening on the closet flange slots to remove the broken part of the bolt. Make sure you scrape off the flange before installing the new wax seal and bolts. Some new wax seals include new bolts in the package, some don't. Check the box when you buy it. Click on the link below for instructions. The only part in this link that I disagree with is placing the wax seal on the bottom of the bowl. It is likely to be damaged while setting the toilet so I place the seal on the flange and look through the holes on the bowl to properly align when setting it.
Installing A New Toilet