In order to secure your blog from potential hackers, it is good to hide the wordpress version appearing within the <head> and </head> html tag of your blog. To Hide the wordpress version just open functions.php file of your active theme and add this code:
Since WordPress 2.5 version is inserted automatically. !Once you add this code in function file, it will remove wp_generator() function which prints the WordPress version. As this function will not executed your WordPress version will not be visible in source code.