How do you remove the license plate on a 2005 Hyundai Sonata?

I just bought a 2005 Hyundai Sonata from my In-Laws, and we transferred the plates over, but I can't get their old plate off? There is this funny white thing on the left hand side, that just keeps spinning. How do I get it off and put mine on. I don't want to get pulled over for it.|||Open the trunk or behind the bumper.... hold the back of it and then twist.