You can use javascript to access elements on the page and modify their contents. So for example you might have a page with some HTML markup like so:
<div id="MyEdit">
This text will change
</div>
You can use javascript to change the content like so...
document.getElementById("MyEdit").innerHTML = "My new text!";
Here is a working example
You can also look at using the JQuery javascript library for DOM manipulation, it has some great features to make things like this very easy.
For example, with JQuery, you could do this to acheive the same result...
$("#MyEdit").html("My new text!");
Here is a working example of the JQuery version
Based on this example you provided in your post. The following JQuery would work for you:
var x = "hello wolrd";
$("p").html(x);
Here is the working version
Using a P tag like this however is not recommended. You would ideally want to use an element with a unique ID so you can ensure you are selecting the correct one with JQuery.
innerHTMLis still the easiest way. You can also create a text node and append it to the target node.