The seal on either side of the bearing is just an interference fit. It can be popped out using a very small screwdriver, dental-type pick or even a thin knife blade. The important detail is not harming the seal when doing this. Pop out via the outer (larger) edge, as this edge does not rotate about the outer race when assembled. The inner (smaller) edge does rotate about the inner race and must not be scarred or you'll need to replace the bearing.

After thoroughly cleaning out the bearing with aerosol parts cleaner (with the little plastic tube), the bearing innards need to be dried completely before re-packing with a high quality wheel bearing grease. Then the seal simply pops back into place. Best to rotate the bearing and press the seal edge repeatedly to ensure its proper placement.

Here's a pretty good video describing the process. Clean the innards out well and dry, don't just re-pack. The gear lube is a contaminant at this point.
Lubricating sealed bearings