The bearings are pressed into the hub and spindle assembly setup. You better have a good hydraulic press and the proper bearing pullers for the setup. If you don't, then it might be better if you have a complete hub assembly which isn't the cheapest way to fix this. What's the reason for the replacement? Is there too much bearing play or noise? If no then just leave them alone until they start to fail. I've never seen one of these fail but if your car is tracked a lot, then the stress on the bearings is definitely amplified. For a track car, I would buy a second set of assemblies for a quick replacement when needed, and then you can rebuild the old units at will. Good luck.
Al