можно так
<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>test</title>
</head>
<body>
xxxxxx<br>
Test phrase<br>
xxxxxxxxxxxxx
<script>
document.body.innerHTML = document.body.innerHTML.replace('Test phrase', 'Test PHRASE');
</script>
</body>
</html>