There are several methods of cleaning bearings some are a pain in the ass but effective, others are easy but not so effective;
1. Pry out the side cover from one side of the bearing so it exposes the balls, blast with motor spray. Install the bearing with open side on the inside of the hub. Most effective way of cleaning but bearing can die quicker!
2. Buy a bearing/glasses cleaner (one of those ultrasonic vibration things!) soak bearing in thinners and vibrate.
3. This is one of my favourite! Drip bearing oil onto one face of the bearing, spin the bearings, use a tissue to clean. Keep repeating until the oil flowing through the bearing is clean.... this can take forever but it leaves very clean, lubricated bearings!
On each one of the above, don't forget to lube afterwards with bearing oil! The easiest method is probably 2. but does not get rid of all the dirt.