You should receive automatic update notifications (after a while) in your WordPress. If you don’t, then your WordPress’ updates may be blocked or malfunction. Are you able to install new extensions through the dashboard?
Once a theme is installed you cannot use the zip file to update it directly. You need to first delete the theme (you’ll have to temporarily activate a different theme to do this), then you can (re)install it using the zip.
With Fluida the theme options are kept during this procedure.