You are looking for CV29, the NMRA standard configuration CV. Digitrax has a good summary of how to set CV29:
http://www.digitrax.com/v1/cV29%20config%20register.htmIn general, since Digitrax designed the EM13, you will probably find that Digitrax references may help you find otherwise undocumented features of the decoder. (That said, I do not believe that the EM13 supports 14-step or 28-step tables; but I haven't tested mine to be sure.)