I just changed all 4 tires on my JLC Challenger, so here are some pointers:
The tires are all on the last drivers on each engine set, so you'll have to take the screw out that holds the main rod and eccentric on all the wheels with tires. There's enough slack to lift all the rods up and slip the tire under.
The rear engine has the last wheel set right up near the firebox. You'll need a couple pics or small screwdrivers to help guide the tires on.
Remove the brake details near the tire you're working on to get the tire on easier.
Make sure you put the eccentric back how you found it. Take a picture so you put it back how it was when you removed it.
All in all it's an easy job. Hardest part is getting the new tires on straight. Should take you maybe 10 minutes or so to do.