You can update this PHPAuction setting through the script's MySQL database. The most convenient way to manage the database is through the phpMyAdmin tool. Go to your cPanel and click on the phpMyAdmin link in the Databases box.

Once in the phpMyAdmin select the PHPAuction database from the drop-down menu on the left. The page will refresh and the database's tables will be displayed on it. Open the SQL tab (look at the top navigation bar).

In the text field write the following SQL query:

UPDATE `PHPAUCTION_settings` SET `siteurl` = "http://servername/~username" WHERE `PHPAUCTION_settings`.`siteurl` = "domain.com";

Change "servername" to the server's hostname you account resides on and "username" to your cPanel user. The "domain.com" stands for the domain name the PHPAuction is initially setup.