To create the chamfer: draw the shape of the chamfer using a polyline in model space.
Right Click on the polyline and create a new profile with a name that makes sense.
Go into style manager and in the architectural section find slab edge styles.
Create a new one and give it a name that makes sense.
Edit the new style and goto the Design Rules tab.
Change the Fascia or Soffit to the profile that you created and make any other necessary changes. Save and close.
Goto the slab that you want to change and select the slab. Right click and select Edit Slab Edges and then pick all of the edges that you want to change (only the ones that you pick show up in the list). Change the Edge Style to the new edge style that you created. You may also need to change the overhang to a negative value. ie. -.5"