<html>
<head>
<script>
function check()
{
document.getElementById("check1").checked=true
}
function uncheck()
{
document.getElementById("check1").checked=false
}
</script>
</head>
<body>
<form>
<input type="checkbox" id="check1">Do you like summer?
</form>
<button onclick="check()">Check Checkbox</button>
<button onclick="uncheck()">Uncheck Checkbox</button>
</body>
</html>