materio-d8/web/modules/custom/materio_commerce/materio_commerce.module

<?php

function materio_commerce_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'commerce_checkout_flow_multistep_default') {
      if ($form["#step_id"] == "review") {
          $t="t";
        // $form['buttons']['continue']['#value'] = t('Desired Custom Text Here');
        $form['actions']['next']['#value'] = t('Place your order');
      }
  }
}