HOW TO TURN FUNCTION OFF AND ON

I would like to turn a php function on and off, I’ve placed this code on my index.php

<?php
function default_sort_ascend($t) {
     $q = $t->query_vars;
     if ( empty($q['order']) || ((strtoupper($q['order']) != 'DESC') && (strtoupper($q['order']) != 'ASC')) )
          $q['order'] = 'ASC';
     $t->query_vars = $q;
}
 add_action('pre_get_posts', 'default_sort_ascend');
?>

How to create a button to switch back and forth from ‘DESC’ to ‘ASC’?

The code above is great because it affects not only the home page but also the archives, etc…

Thank you