You must convert the database struckture of sFS 1.2.1 to be able to upgrade to use sFS 1.3.
Please note that its always riskily to make changes to a database struckture wich contains data so its recommendet to make a backup before starting the update script.
Im not responsible for any damage wich could be caused by the updating script, if didnt make any backups and something goes wrong your site is gone forever.
Upload the whole /page and /backend folder to your root directory and replace the old php files.
Warning: Dont replace the template folder if you have a custom design. If thats the case you must adapt the your custom templates to be compatable with the new version (Please note that i cant support you with that).
After the upload is finished point your browser to the install folder in the backend folder and open the update.php
i.e.: http://simple-fs.com/backend/install/update.php
The update script will print a warning message, you must agree to proceed the update process.
After some seconds the updating should be complete and you will see a message from the script.
After the updating process is done you should login into the admin panel and look if the offer page settings and everything else is ok.