If you are upgrading, you need to update your current installation's
database by browsing to [URL]/setup.php where [URL] is the
postfixadmin webroot.

To avoid checksum errors when removing or upgrading postfixadmin, do not
edit config.inc.php. Instead put your configuration settings in
config.local.php in the postfixadmin webroot. That file will be included
automatically by config.inc.php.

For detailed information, please see the complete installation steps in
%%DATADIR%%/INSTALL.TXT