At first you must save your mysql database. The following command will save the database ocsweb :
mysqldump -u root -p ocsweb > ocs-database-save.sql
Next you must save the files ocsinventory-server.conf and ocsinventory-reports.conf that you will find in the apache config directory.
after, simply run the setup.sh that you will find in the tar.gz you downloaded on http://www.ocsinventory-ng.org. It will overide the currently installed version. After having install it, go on ocsreports and launch the update procedure for the database.
After update, thinks to change your mysql access credentials in ocsreports and in the server.