Set permalink to post after theme activated in WordPress

reset_permalinks.php

<?php

/*----------------------------------------------------
Set permalink to post after theme activated
-----------------------------------------------------*/

add_action('after_setup_theme', 'reset_permalinks');
function reset_permalinks()
{
    global $wp_rewrite;
    $wp_rewrite->set_permalink_structure('/%postname%/');
    $wp_rewrite->flush_rules();
}