functions.php

/**
 * Change the Shop archive page title.
 * @param  string $title
 * @return string
 */
function wc_custom_shop_archive_title( $title ) {
    if ( is_shop() && isset( $title['title'] ) ) {
        $title['title'] = 'My Title';
    }

    return $title;
}
add_filter( 'document_title_parts', 'wc_custom_shop_archive_title' );