Hi JEllington,
Please try the following scripts below and this also works on asynchronous deployments:
In the Header of your DataPage:
<div id="elmName">Original Text</div>
In the Footer of your DataPage:
<script>
document.getElementById("elmName").innerHTML = "Text changed!";
</script>
Then click "Preview" to check.
Note: Please make sure you only have one id of elmName declared in your page.
Let me know if this helps.