Cant change the style of my site with Javascript
<p>I am a beginner at Javascript and been sitting with this problem for a while, somewhere i've done something wrong, but I don't think I have understood the concept about it.
I am trying to call a function from when i click my button, however i can do this easily with alert and prompt, works perfectly but when im trying to change the site of my website, it does not work. Would be happy if you could tell me where I am doing wrong and why please.</p>

<pre><code>window.onload = saved;

function ChangeStyle(){
    var ThirdButton = document.getElementById('buttonThree');
    ThirdButton.onclick = ChangeText;
}
function ChangeText(){
    p.innerHTML = "CantCareLess";
}

function saved()
{
ChangeStyle();
}
</code></pre>

<p>HTML</p>

<pre><code>&lt;button id="buttonThree"&gt; Utseende &lt;/button&gt;
</code></pre>

<p>So i have plenty of text on my website, with heaps of paragraphs.. However, when i click the button it doesn't change all the paragraphs to "CantCareLess", it doesn't change it anywhere..</p>

<p>If i put in alert("Hello"); in my function, it works perfectly, so there's nothing wrong with my function.</p>