Before you do anything get a service manual.
Pull off the right side engine cover and remove the clutch with the oil pump. Take off the left side sprocket cover. There may be a circlip on the shaft at the seal - can't remember off the top of my head. If there is remove it. Then disengage the rachet pawl from the shift drum and slide the shaft out from the clutch side. Slide the new shaft in and replace the seal while you're at it.