Knowledge is assumed of LAMP and Asterisk for this guide. The upgrade procedure is not dissimilar to the installation of A2Billing as described at http://www.asterisk2billing.
Upgrading an existing installation is straight forward, but before starting, backup the current A2Billing files and backup the A2Billing database.
Download (http://www.asterisk2billing.org/get-started/download/) and explode the tarball into the location of the A2Billing directory. Replace the Admin, Customer, Agent and Common directories with the latest version and set the appropriate permissions according to your webserver configuration.
Replace a2billing.php with the new version and update the common/lib directory.
Replace a2billing.conf with the new version and adjust to suit your database and other settings. Also set the csrf_token_salt value to a random alpha-numeric value.
Apply each “DataBase/mysql-5.x/UPDATE...” in order, from the version after the one you are on up to the current version.
Copy any new IVR files from addons/sounds to the Asterisk sounds directory.
Update the cronjobs with the latest versions and check they will run.
You should now be running the latest version of A2Billing.
Bear in mind that running any kind of telephony server on the open internet makes you a lucrative target, and as such, you should ensure that you have taken all practical steps to secure your installation.