Undo with UpdatePanel

I want to reset 2 textboxes that are inside UpdatePanel to original values when Undo is clicked. It doesn't work because of UpdatePanel. When user changes the zip code it retrieves a corresponding state asynchronously which causes ASP.NET to automatically stores the new instead of old values in _VIEWSTATE. So when I click Undo, it uses the new value in _VIEWSTATE to undo and that's not correct. Can someone show me how to make it work? Thank you.
Some people suggested to call Page_ClientValidate(); in reset button code but I don't know how to implement that. Is there an example?
 
<input type="reset" id="ResetBtn" value="Undo" runat="server" onclick="undoFields();" /> <input type="hidden" id="zipHidden" value="" runat="server" /> <input type="hidden" id="StateHidden" value="" runat="server" /> <tr> <td>Zip Code</td> <td> ...