JSFiddle - React, Tailwind, and code Playground

HTML

<select id="s">
    <option>Cat</option>
    <option>Mat</option>
</select>
<button id="b">De-select</button>
<hr/>
<select id="s2" multiple>
    <option selected>Cat</option>
</select>
<button id="b2">De-select</button>

JavaScript

document.getElementById('b').onclick = function() {
    var sel = document.getElementById('s');
    sel.selectedIndex = -1;
};

document.getElementById('b2').onclick = function() {
    var opts = document.getElementById('s2').options;
    for (var i = 0, opt; opt = opts[i]; i++) {
        opt.selected = false;
    }
};