How can i change email template for new user

For 2018 and onwards users:

David Gard’s answer still works but is old and there’s a new better/cleaner way to do this (no need for a plugin anymore).

Since WordPress 4.9.0 there are new filters you can use to customise registration emails:

• wp_new_user_notification_email – customise email sent to User
• wp_new_user_notification_email_admin – customise email sent to Admin

Usage example on email sent to Admin (you can paste this in your theme’s functions.php ):

add_filter( 'wp_new_user_notification_email_admin', 'custom_wp_new_user_notification_email', 10, 3 );

function custom_wp_new_user_notification_email( $wp_new_user_notification_email, $user, $blogname ) {
    $wp_new_user_notification_email['subject'] = sprintf( '[%s] New user %s registered.', $blogname, $user->user_login );
    $wp_new_user_notification_email['message'] = sprintf( "%s ( %s ) has registerd to your blog %s.", $user->user_login, $user->user_email, $blogname );
    return $wp_new_user_notification_email;
}