Replace all elements with the selector 'pmpro_btn' to instead use the selector 'button'.

my_pmpro_element_class_buttons.php

<?php
/**
 * Replace all elements with the selector 'pmpro_btn' to instead use the selector 'button'.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 *
 */
function my_pmpro_element_class_buttons( $class, $element ) {
	if ( in_array( 'pmpro_btn', $class ) ) {
		$class = array();
		$class[] = 'button';
	}

	return $class;
}
add_filter( 'pmpro_element_class', 'my_pmpro_element_class_buttons', 10, 2 );