The Update button is actually intentionally not there. The direction that the Themes section was to take was something similar to how Modules was setup. So that you had one entry for each theme with many versions. This would allow users to go back and forward in a themes history to avoid problems etc. When the section gets rewritten here shortly this should be implemented and you will understand. As for finding all of your themes...Go into the themes section and browse by author. Its the best we can do right now.