garage door replace

There is a gear wheel at each side of the which runs in a track. The cable is attached to these. I had to remove the cable at one side to replace the gear spindle and managed to remove cable and refit without any hard work? However i need to replace the cone how hard of a job is this? Is it just a case of knocking out the old roll pin and fitting the new cone back into the same place? Any help please preferbly before i do the job there is nothing on the net about this? but then again that is always the way.