What Exactly U want?
onChange event of Textbox add hyperlink to the Page or Change current hyperlink?
Both the things are easily done through client side scripting..
If u want to add new links as per ur textbox then..
Code:
<div id="divLinks" runat="server">
<a href="xyz1">Link1</a>
<a href="xyz1">Link1</a>
<a href="xyz1">Link1</a>
</div>
Code:
function addNewLink(str)
{
document.getElementById("divLinks").innerHTML = document.getElementById("divLinks").innerHTML + "</br>" + str
}
if you want to change the link then refer it's id and set href value.