CodingForums.com - Could someone help me how to convert this to form action

So I have follow code. Which replace username "333" to "reer" in database.
But I'm kinda lost how to make this code working follow method:

There is field box where typing user's current username and second field where type new user's username and then just submit. If someone have time to help me I appreciate it very much. :)

Code:

<?php

$con = mysql_connect("localhost","root","");

if (!$con)

  {

  die('Could not connect: ' . mysql_error());

  }



mysql_select_db("database", $con);



mysql_query("UPDATE users SET username='reer'

WHERE username='333'");



mysql_close($con);

?>