Download the latest update (zip format)... unzip it on your computer. Then, upload ALL the files to your server (make sure to make a back-up copy of your server files
BEFORE uploading). If you made any changes to the "core" files or to the Modern theme, they will be overwritten (so make a backup of any files you wish to save). However, any additional themes or plugins will be
safe, including your database.
If you have permalinks enabled, you should disable it before updating OSClass. Then,
update your installation by visiting
http://example.com/oc-includes/osclass/upgrade-funcs.php. If a database upgrade is necessary, OSClass will update your database to be compatible with the last code.
That's it.