You will need to chisel out the slab from around to riser to the toilet to give yourself enough room to insert a hubless connector to connect the riser to a proper flange. You may have to chip away a bit of concrete to accomplish this, and you may have to cut the riser to allow proper placement of the flange at just above the floor level. Special pipe-cutting tools are available for this latter procedure, but they are costly. You should be able to hire a competent plumber for less than the cost and aggravation of doing the job yourself.
NtP