Add the following code to the web form page designer: 
  1. Click Web > Page Designer 
  2. Click Next until you get to Configure Content 
  3. Click the wheel icon for the header content 
  4. Click the View tab 
  5. Click the HTML Code button 
  6. Paste the following code at the bottom of the page: 
  7. <script type="text/javascript">// <![CDATA[
    $( document ).ready(function() {
        if($(".RegisterLinkButton").text() == 'Register') {
            $(".RegisterLinkButton").text('Create an Account');
        }
    });
    // ]]></script>
  8. Click Update
  9. Click Save
  10. Click Submit