How do you replace the large gear?

How do you replace the large gear?

My Protocol Dronium Two has some stripped teeth on one of the large gears that mesh with the small motor gear. I have got a replacement gear (and shaft). Does the gear pull off the shaft? and how hard?? Seems very tight. Does the shaft need to be replaced also, it does not seem to be damaged. Not sure how hard to pull on the gear?