To change the database prefix to prevent SQL Injection Attack (if not done during install)

Backup your database and site first.

Method 1 - PHPMyAdmin (inbuilt)

Newer version of PHPMyAdmin have a table prefix changer inbuilt and is now the prefered method.

• Open up your Joomla database in PHPMyAdmin
• Check All tables. If you can see them all exand the number of tables you can see on the page.
• At the bottom, With Selected: , choose Replace table prefix
• It will now ask you for the From prefix which is your current one jos_ , you should fill this in the box. You can also usually see the database prefix in the column on the left. The To prefix should be a random 5 characters using upper case, lower case and numbers (ie TrFgH_).
• Click submit
• Done

Method 2 - Old Text File method

Taken from here

• Make a note of you database prefix either from joomla itself or the configuration.php file. For this example we will assume jos_
• Go to PHPMyAdmin and export your database as a SQL file
• Open database SQL file in a text editor (Notepad++).
  • Search and replace (Ctrl+h in Notepad++)  jos_ with your new database prefix, it should be a random 5 characters using upper case, lower case and numbers (ie TrFgH_).
  • Save the file
• Go in phpMyAdmin
  • select your Joomla database and empty it of tables and data (i.e. Truncate)
  • import the new SQL database file from above in to your Joomla database
• Via ftp edit your configuration.php to refelct the new table prefix. No other details will need to be changed.
• Done