Change the "Change" link to "Upgrade" on the Membership Account page under My Memberships.

customize-change-link.php

<?php
/**
 * Change the "Change" link under My Memberships to "Upgrade"
 *
 * 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 customize_change_link_for_pmpro_member_action_links( $pmpro_member_action_links ) {

	$pmpro_member_action_links['change'] = sprintf(
		/* translators: 1: Membership Levels URL. 2: Link text. */
		'<a id="pmpro_actionlink-change" href="%1$s">%2$s</a>',
		pmpro_url( 'levels' ),
		esc_html__( 'Upgrade', 'paid-memberships-pro' )
	);

	return $pmpro_member_action_links;
}
add_filter( 'pmpro_member_action_links', 'customize_change_link_for_pmpro_member_action_links' );