How to update the client ASP.NET VB variable?

I have registered ASP.NET VB variable into the client during page-load using code below:
 
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load Dim csm As ClientScriptManager = Page.ClientScript Dim hiddenName As String = "HiddenCartProdCode" Dim hiddenValue As String = “InitialValue” csm.RegisterHiddenField(hiddenName, hiddenValue) End Sub I want to change the hiddenValue = “InitialValue” to become hiddenValue = “100” when user clicks ASP control button (during post-back). What is the code to update the hiddenValue?
 I am using VS 2008. Thanks.
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