This is easier to do before you install WordPress. Just make the change in the wp-config.php file when you enter the database info. There are a few plugins that you can use to make the changes after WordPress is installed.
Originally Posted by Oranges
If you change the prefix manually after installing WordPress, then you will have to manually make changes to several table rows. Otherwise, WordPress will not work.
WordPress › Support How to change table prefix
An additional important step is to always change the admin username from 'admin' to something that is not easy for a hacker to guess. This one is easy, just open the users table using phpMyAdmin and change the user_login field in the first row.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin