If you have to change your textbox value using javascript if you are using update panel you can use the following code for changing the value and before doing that check the following instructions.
1. Makesure that you are returning false after assigning the new value to the textbox if you are not returning anything your assigned value in the textbox will replace to the previous value.
2.Download the new jquery library from here
http://code.jquery.com/jquery-1.10.2.min.js[
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Use your code like this
Sys.WebForms.PageRequestManager.getInstance().add_initializeRequest(function () {
$("<%=Button1.ClientID").click(function(){
document.getElementById("<%=TextBox1.ClientID %>").value = "hello";
});
});
<asp:button id="Button1" runat="server" text="Click" xmlns:asp="#unknown">
<asp:textbox id="TextBox1" runat="server" xmlns:asp="#unknown">
And add the same code inside the document.ready function