Hey!
When I assembled our Banebots planetary to work with our Vexpro motors, I used a bench vice to attach the pinion gear onto the motor output shaft. Although I probably wouldn't recommend a bench vice, if it's what you have available, then it's probably your best option. A arbor press is most likely the best method for a task such as this.
Remember when you're pressing the gear onto the shaft that everything is level and straight, and that you are supporting the end shaft.
Just be careful you don't press the pinion gear on too far, just put it on far enough so it's fully on the shaft end, but not all the way down.
There's a little video on a similar gearbox from banebots, I used it as a reference.
https://www.youtube.com/watch?v=ZLLQXDcXAKE
As for taking off the other gear, I would recommend a gear puller, or if you don't have access, two screwdrivers to pry it off. Just be careful not to mess the gear teeth up.