I have 10/3 wire. Can I replace my 3 prong outlet with 4 prong for a dryer?
First, separate neutral and ground at the receptacle, that should never have been done.  Paralleling neutral is not allowed.   

Second, go back to the service panel and make sure the neutral is on the neutral bus... And the ground is on the ground bus.   Inside a main panel ("service equipment" as NEC describes it in RME's post above) don't be surprised if they use the same bar for neutral and ground.  That is wrong to do in a subpanel. 

Now install the new receptacle in the normal way.   Hats off to the last guy for fitting the right cable.