<script type="text/javascript">
var buttonRenewal = document.getElementById('ctl00_ContentArea_MemberProfile1_Membership_RenewalButton');
if (buttonRenewal) {
buttonRenewal.value = 'Text you would like to appear on button';
}
</script>
If you want to simply change the text to something constant, you can insert the above code into Global Javascript. Any more functionality would require additional Javascript though.