Follow the following procedure:
– Backup your database and verify it is a complete backup
– Close your board
– goto: http://www.myboard.com/install/tableprefix.php
– Follow the instructions
– When finished, edit your ./includes/config.php with the new prefix.
– Reopen board
Note: Do not alter the tableprefix value in config.php until this script has completed and you are given a message telling you to edit the value.