If they are standard cup-and-cone hubs (which they probably are) the procedure is the same no matter what country they came from and is nicely laid out as follows: http://www.sheldonbrown.com/tooltips/hubs.html

I've never bothered replacing a cone before. Replace the balls, as long as the cones don't have any serious gouges in them they'll be fine. Just adjust the hub as best you can.

If they are cartridge bearing hubs (I don't like 'sealed' because the word causes endless confusion) then there is really no maintenance to do, just replace the whole bearing when they go bad. If you want you could probably get some oil in there to lube them.