Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

Checkbox: <input type="checkbox" id="myCheck">

<button onclick="check()">Check Checkbox</button>
<button onclick="uncheck()">Uncheck Checkbox</button>

<script>
function check() {
    document.getElementById("myCheck").checked = true;
}

function uncheck() {
    document.getElementById("myCheck").checked = false;
}
</script>

</body>
</html>