You should backup the osticket directory on the webserver and the osTicket mysql database.When you have your custom theme or other settings laying apart from the osTicket directory / database, then I'd backup that too.Regarding upgrade itself see:http://forum.osticket.com/d/discussion//resolved-upgrade-to-latest-release#latest

"but i can't lose my theme"osTicket is not themeable at this time.  This makes me think that you have performed customized changes to the source code or installed a 3rd party modification (theme).  You would want to contact the author of that mod and see if they have a version available for the version that you are upgrading to.