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Display modified confirm on the button submit from button action
<ajaxToolkit:ConfirmButtonExtender ID="cbe" runat="server"
   TargetControlID="Cancel"
           ConfirmText="Are you sure you want to click this?"
         />

public void display()
{
   string error="my error";
   cbe.ConfirmText = error;

}

protected void Cancel_Click(object sender, ImageClickEventArgs e)
        {   display();
            anothermethod();
            Response.Redirect("home.aspx");
        }

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        string error = "my error";
        cbe.ConfirmText = error;
    }
}