If you create a password protected page in WordPress, you have the standard sentence: "My post is password protected. Please ask me for a password:". To change this sentence without losing the changes after the next upgrade, just place into the functions.php of your theme the following code:

function fb_the_password_form() {
	global $post;
	
	$label = 'pwbox-'.(empty($post->ID) ? rand() : $post->ID);
	$output = '<form action="' . get_option('siteurl') . '/wp-pass.php" method="post">
	<p>' . __("My post is password protected. Please ask me for a password:") . '</p>
	<p><label for="' . $label . '">' . __("Password") . ' <input name="post_password" id="' . $label . '" type="password" size="20" /></label> <input type="submit" name="Submit" value="' . esc_attr__("Submit") . '" /></p>
	</form>';
	
	return $output;
}
add_filter('the_password_form', 'fb_the_password_form');

Just change the sentence in this code to your liking. That's all. 🙂