The following CSS code will alter the "Renew" link of your Paid Memberships Pro Account page and make it into a button.

pmpro-create-button-for-renew.css

/**
 * The following CSS code will alter the "Renew" link of your
 * Paid Memberships Pro Account page and make it into a 
 * button. 
 *
 * If you do not have an expiration date set for
 * your membership level and thus no "Renew" link, the first
 * link will be affected by the CSS. In most cases, this
 * will then be the "Change" membership link.
 *
 * Add this CSS to your WordPress Customizer's Additional CSS section 
 * (WP Dashboard > Appearance > Customizer > Additional CSS)
 * Code Recipe written by Paul Barthmaier
 * https://gist.github.com/pbrocks
 */

#pmpro_account-membership > table > tbody > tr >
td.pmpro_account-membership-levelname > div > a:nth-child(1) {
	background: rgba( 250, 128, 114, 1 );
	border: none;
	border-radius: 5px;
	box-shadow: 0px 1px 3px rgba( 0, 0, 0, 0.3 );
	color: #FFF;
	font-weight: 700;
	margin-right: 1rem;
	padding: .3rem 1rem;
	text-decoration: none;
	transition: all .4s;
}
#pmpro_account-membership > table > tbody > tr >
td.pmpro_account-membership-levelname > div > a:nth-child(1):hover {
	background: rgba( 250, 128, 114, .3 );
	box-shadow: 0px 1px 3px rgba( 0, 0, 0, 0.5 );
	color: rgb( 250, 128, 114 );
}

This recipe is included in the blog post on "Increase renewals by changing the “Renew” text link into a button on your Membership Account page." at Paid Memberships Pro here: https://www.paidmembershipspro.com/increase-renewals-by-changing-the-renew-text-link-into-a-button-on-your-membership-account-page/