// set text and navigate url property of hyperlink field using jacascript

var hyperlink = document.getElementById('<%= Hyperlink.ClientID %>');
hyp.innerHTML = 'My New Link';
hyp.href = 'http://snippets.dzone.com/'

Read more: http://feeds.dzone.com/~r/dzone/snippets/~3/Y6r2-FieP34/14549