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The wheel bearings are pressed in to the drum.
Remove the wheel. then remove the big nut in the centre of the drum/hub assembly, then the drum can be removed, you may need to give it a few hits with a hammer to help remove it.
Once off, the wheel bearing is held in with a circlip at one end, then the whole bearing assembly needs to be pressed out of the drum in the direction of the circlip using a hydraulic press.
It will take between 5 and 10 tons of pressure to push the bearing out, once removed, clean out the hub, then press the new bearing into the hub and replace the circlip.
Hope this helps, if you need more information, please let me know.
generally yes, it is a garage job, however if you remove the hub and take it along with the new bearing to a garage, they may press the old ones out and the new one in for about £5-10.