Changing anchor text with JavaScript

Assuming the video-quality div will only ever have one anchor in it, you're not far off:

var vid_id=document.getElementById('video-quality');
var anchor=vid_id.getElementsByTagName('a');
anchor[0].innerHTML="High Quality";
//    ^^^-- change here

(There's a shorter way if you don't need to support IE7 and earlier, see David's answer for more.)

The key was in the name getElementsByTagName (note the plural). It returns a NodeList, not an element. A NodeList is (shockingly!) a list of matching nodes, in this case matching elements. innerHTML is a property of individual elements, so we have to index into the NodeList to access a specific element. The code above assumes there is at least one matching element, and updates its HTML. If you want to be more defensive:

var vid_id=document.getElementById('video-quality');
var anchors=vid_id.getElementsByTagName('a');
if (anchors[0]) {
    anchors[0].innerHTML="High Quality";
}

That allows for the possibility there are no matching anchors.

Note that by doing this, you're removing the span element, since the span is a descendant of the anchor. If you wanted to avoid that, just look for span elements as descendants of the anchor, exactly as you've looked for anchors in the div.

FWIW, I'd recommend using a reasonable JavaScript browser-focussed library like jQuery, YUI, Closure, or any of several others. They provide a lot of utility functionality, and smooth over some browser differences. But if you prefer using the DOM directly, that's fine too.

If you don’t need legacy support (IE7-), you can write:

document.querySelector('#video-quality a span').innerHTML = 'High Quality';

If you DO need legacy support, I recommend a library like jQuery or similar to make selecting DOM nodes much simpler from a singe, wide-spread API.

Plain javascript:

var vid_id = document.getElementById('video-quality');
var span = vid_id.getElementsByTagName('span');
span[0].innerText='High Quality'

Or you can just use jQuery:

$('#video-quality span').text('High Quality');

hope this helps.

Simple is best. Just give the span element an id and then alter it directly.

<div id="video-quality" style="">
<a class="low bttn" href="">
<span id="quality-setting">Low Quality</span>
</a>
</div>

then script is:

document.getElementById('quality-setting').innerHTML="High Quality";