Sealed bearings aren't really sealed. Sure, they're self-contained and easy to throw away and replace, but what if they're not quite ready to go? Or what if you want to do some preventative maintenance? Or what if they're proprietary to one component, and you won't get a replacement in time for your ride on Saturday? You un-seal them, clean them out and regrease them just like we used to do back when hubs, headsets and bottom brackets used loose-ball cup-and-cone setups.
This won't work if the outside of the seal is aluminum. Those deform too much getting them in and out. But a rubber seal will come out, no problem. Make sure everything is wiped clean, and use a sharp pick or the corner of a razor blade to carefully peel up the outside edge of one of the rubber seals. It won't take much force. Pull the other side off as well, and you can spray out any old grease or built-up grime. Pressure is your friend here. Alternate with an air compressor if you've got one.
Once it's clean and dry, pack in grease and roll the bearing back and forth to distribute it. Clean the inside surfaces of the seals and press them back on with your fingers. They should slip right into place without any tools. Wipe off the excess grease, and you've just bought yourself some time before it's time to buy new bearings.