<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:Button ID="Button1" runat="server" Text="Button" OnClientClick="replaceSelection();return false;" />

<script type="text/javascript">
    function replaceSelection() {
        var target = document.getSelection();
        Console(target.log);
        if (document.selection) {
             //target.focus();
               var txt = target.value;
               var selRange = document.selection.createRange();
               target.value = target.value.Replace(selRange.text, "hello!!!");
        }
     }
</script>