function onLoad_ChangeLabel() {
    // get the value of field in variable
var lblIncomeField1 = Xrm.Page.getAttribute("fieldname").getValue();

 // value not equal to null
        if (lblIncomeField1 != null) {
            Xrm.Page.ui.controls.get("Oldfieldname").setLabel(newfieldname);
        }
}




SOURCE : JUST2CODE.IN
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