This is typical for many post war locomotives.
First - you need the proper tools - a good gear puller (to remove one wheel) and press the drive gear off the shaft.
You also need the proper set of wheel 'cups' that have the proper offset to reinstall the wheels so they are 'quartered' properly.
Wheel cups are quite common, so are gear pullers.
My advise - if you have never done this before - either get someone to do it, or be careful.
bruce