I have a rooted Nexus7 (2012). An update of the OS, going from KitKat to Lollipop, downloaded without my doing anything, but I didn't accept the update. Do I have to root all over to update to Lollipop? If not, what do I need to do to wind up with a rooted tablet running Lollipop? How do I specify the correct 5.0.2 build for my Nexus7?