update thumbnails

I just loaded Mavericks and with it, the new version of iPhoto. When I opened the new iPhoto for the first time, it asked me if I would like to begin "updating thumbnails" or wait. I began the update process but had to place it on hold. Now I can't figure out how to resume the process and iPhoto is filled with a bunch blank squares where my thumbnails should be. Help! How do I finish the process?