Heres a schematic of your motors gearcase
http://www.crowleymarine.com/johnson-evinrude/parts/40695.cfm
Remove the shift shaft connecter from the oval plate on the exhause housing,Drop the lower, turn the lower upside down,remove the phillips head pivot screw,then remove the six screws that hold the lower part of the gearcase with the skeg on it.The shift shaft will then come out for your replacement.
Why are you replacing the dog? If its popping out of F gear, the F gear will most likly be bad also.