How in HTML or Javascript do you replace text in a table with different text?

I want a table say something like "Hello!" until someone presses a button, then I want the table to say "Goodbye!". How would I do this, could someone give me a sample of code, or a link to a website that says what to do?

Let's say you have a very simple table:

<table>
<tr>
<td id="TheMessage">Hello!</td>
</tr>
</table>

And then your button:

<input type="button" value="Say bye" onclick="sayBye()" />

Then your sayBye function would be like this:

<script type="text/javascript">
function sayBye() {
var td = document.getElementById( 'TheMessage' );
td.innerHTML = 'Goodbye!';
}
</script>

well this can vary. you can use Javascript and just run an if statement like this:
That is based off of whether at says logged in or not. doing on onclick you can run a function.
<script type-"text/javascript">
var iuser = document.getElementById("login");
if(iuser.innerHTML.match(/Logged in/i) {
document.getElementById("welcome").inn... ="Welcome"
}
else {
document.getElementById("welcome").inn... ="GoodBye"
}
</script>


The Function:
<a href="javascript:welcomegoodbye();">Say Goodbye </a>
<script type-"text/javascript">
function welcomegoodbye() {
document.getElementById("welcome").inn... ="GoodBye"
}
</script>

so there are a couple ways of doing it.