bohdantheone, для какой цели?
Как вариант...
<!DOCTYPE html>
<html>
<head>
<meta name="Keywords" content="test">
<!--
<script src="http://code.jquery.com/jquery-latest.js"></script>
<link rel="stylesheet" type="text/css" href="tmp.css" />
-->
<style type="text/css">
</style>
<script type="text/javascript">
var o=document.getElementsByTagName('meta')[0];
alert(o.content);
o.content='new text';
alert(o.content);
</script>
</head>
<body>
</body>
</html>