add_action( 'after_setup_theme', 'theme_functions' );
function theme_functions() {
add_theme_support( 'title-tag' );
}
add_filter( 'pre_get_document_title','sm_pre_get_document_title');
function pre_get_document_title(){
global $post;
if( $post->ID != 6486 ) return;
return 'Тест';
}