I found online instructions for updating the firmware on the RDU with the older RTL500, but not with the RTL510. In my experience, with the RTL510 plugged in via USB, the RDU will not connect to it. Thus, the directions from the older RTL500 don't work, where it claims the RDU will pair to the RTL and download any updates from the RTL while it's connected to Garmin Express. Perhaps this still works once new software is downloaded to the RTL510 and later connected to the RDU?
As usual, poor Garmin documentation. You also can't search the forum for "RDU" because Garmin's search filter takes it out because of too few characters... grr.
PS: Shawn-Garmin it'd be awesome if you could cleanup the stickies on the top of this forum. There's several that are 3+ years old and not really relevant anymore.