Are you trying to update label text which is in update panel?
Can you try the below code.
<asp:Label runat="server" Text="salam" ID="lbl" ></asp:Label>
<asp:HiddenField id="hdlbl" runat="server" />
document.getElementById("lbl").InnerHTML = "10"; // update client side
document.getElementById("hdlbl").value = "10";
Add the following code in page load
if(Page.IsPostBack)
{
lbl.Text = hdlbl.value;
}