Attr.value

<label test="initial value"></label>

<button>Click me to set test to <code>"a new value"</code>…</button>
<br><br>
Current value of the <code>test</code> attribute: <output id="result"><i>None.</i></output>

const elements = document.getElementsByTagName("label");
const buttons = document.getElementsByTagName("button");
const result  = document.querySelector("#result");

const attribute = elements[0].attributes[0];
result.value = attribute.value;

buttons[0].addEventListener('click',function(e) {
    attribute.value = "a new value";
    result.value = attribute.value;
  }
);