In your theme’s functions.php:
function my_custom_js() {
echo '<script type="text/javascript" src="https://stackoverflow.com/questions/5849057/myscript.js"></script>';
}
// Add hook for admin <head></head>
add_action( 'admin_head', 'my_custom_js' );
// Add hook for front-end <head></head>
add_action( 'wp_head', 'my_custom_js' );