How do I replace title with my plugin?

Please try this code hope it will solve your problem.

add_filter( 'wp_title', 'theme_custom_title', 20 );

function theme_custom_title $title ) {
    return str_replace('your old title', 'your New title', $title); 
}

// changes the "Enter title here" to "Enter some New title" 
add_filter('gettext', 'custom_rewrites', 10, 4);

function custom_rewrites($translation, $text, $domain) {
    global $post;
    $translations = &get_translations_for_domain($domain);
    $translation_array = array();
    switch ($post->post_type) {
        case 'videofaqs':
            $translation_array = array(
                'Enter title here' => 'Enter Video question here'
            );
            break;
    }
    if (array_key_exists($text, $translation_array)) {
        return $translations->translate($translation_array[$text]);
    }
    return $translation;
}