As you know we can not access controls in web method, so You need to use JavaScript/Jqury to set label value.
//first you need to call javascript and then call web method from it, see below snippet
function callwebmthod()
{
PageName.MathodName(Param1, Param2, MathodName_CallBack);
}
function MathodName_CallBack(response)
{
var szReturnFromWebmethod = response.value;
document.getElementById("label1").innerHTML = szReturnFromWebmethod;
}
[Ajax.AjaxMethod()]
public string MathodName(string Param1, string Param2)
{
}
'value' property is not able to set value to label, you nee to use 'innerHTML' property, see below snippet
document.getElementById("label1").innerHTML = "change in text or whatever";
document.all("label1").innerHTML ="change in text or whatever"