Web Forms :: How To Change The Hidden Control Value

i am using <input type="hidden" id="hiddenValue" name="hiddenValue" runat="server">

hidden control in my asp.net, and setting it value when page load, and when i clicked on a button then my server side code change hidden control value, and when i tried to access it value after another clicking of button then it showing me

first time value,

so why hidden control does not reset its value, how can i reset the hidden control value?

My Code

Page_Load(){
if(!Page.IspostBack){
hiddenValue.Value ="ABC";
}
}
ButtoClick1(){
hiddenValue.Value ="BAD";
}
ButtoClick2(){
// it give me 'ABC' value, instead of 'BAD'
string s = hiddenValue.Value;
}