First go to your cPanel and click on the phpMyAdmin link from the Databases box.

From the drop-down menu on the left choose the phpBB2 database. Once the page refreshes with the database contents, open the SQL tab at the top. In the text field write the following SQL query:

UPDATE phpbb_config SET server_name = "servername/~username" WHERE server_name = "domain.com";

You should change "servername" with the server's hostname your account resides on and "username" with your cPanel username. "domain.com" stands for the domain name the phpBB2 was initially installed with (usually it is your primary domain name).

Click the GO button to execute the query. If everything goes without errors, the phpBB2 database will be updated. Now you can access your phpBB2 installation through the temporary server URL.