Hello !
These bearings are not ball bearings, so you can remove the inner race without destroying anything (unless your bike is an LS).
To do so, remove the wheel, then remove the spacer onto which the lip seal lip rolls, this has to be done carefully in order not to mar the seal, then with a large and flat screwdriver, remove the lip seal, using the screwdriver as a lever, a little bit at a time on the circumference, then you can remove the inner race to clean it and repack it with a good bearing grease. Installation is made by reversing the above procedure. Once done on one side, you've to do the other one ;-)
This should be done at every tire change. but YMMMV