Change class for PMPro Level Select button.

pmpro_level_select_button.php

<?php

// Add this function to your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/


function change_pmpro_level_button_class() {
?>
  <script type="text/javascript">
    jQuery(document).ready(function(){
      jQuery('.pmpro_level-select a').attr('class', 'button'); //add 'button' class to level select buttons.
    });
  </script>
<?php
}

add_action( 'wp_footer', 'change_pmpro_level_button_class' );