The value property of the HTMLOptionElement interface represents the value of the <option> element as a string, or the empty string if no value is set. It reflects the element's value attribute, if present. Otherwise, it returns or sets the contents of the element, similar to the textContent property.
A string containing the value attribute value, if present, or the contents of the element.
const optionElement = document.querySelector("datalist option:first-of-type");
const oldValue = optionElement.value;
optionElement.value = oldValue.toUpperCase();