Catch Everest Pro Update

Hi everyone! I just need to clarify theme update procedure. I have customised the website, but only through the theme options and custom css. If I follow the process for updating within the dashboard, i.e. activate an alternate theme, delete the Pro theme and then reinstall the new Pro theme... Will that remove the customised theme options? And if not, how does WP retain all of that info if I have deleted the theme?? Thanks in advance! Paul