Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<head>
<style>
table, th, td {
    border: 1px solid black;
}
</style>
</head>
<body>

<h3>A demonstration of how to access a TH element</h3>

<table>
 <tr>
   <th id="myTh">Name</th>
 </tr>
 <tr>
   <td>John</td>
 </tr>
</table>

<p>Click the button to change the text of the TH element.</p>

<button onclick="myFunction()">Try it</button>

<p id="demo"></p>

<script>
function myFunction() {
    var x = document.getElementById("myTh");
    x.innerHTML = "New Table Header";
}
</script>

</body>
</html>