How to change wordpress post title?

add_filter('the_title', 'wordpress_title');
function wordpress_title($title){

    //Return new title if called inside loop
    if ( in_the_loop() )
        return 'New title';

    //Else return regular   
    return $title;

}

Have you tried the in_the_loop() conditional check to return new title only if called inside loop. That means nav menu’s will not get affected.