How do you change hubs on a bmx bike?

Answer 1

There's usually not much point in changing hubs, as that requires tearing the wheels completely apart - spoke by spoke. and unless the flanges of the new hubs are within mm of the same size the old spokes won't fit. Often cheaper simply to buy new wheels.

But if you insist: remove every spoke from the hub + rim , then lace up the new hubs to the old rim, tension and true.