How to change a single users details in Mysql and php

Send an SQL query like:

update users set Intro=2 where id=1;

This?

UPDATE users SET intro = 1 WHERE id = 2

UPDATE users SET Intro = 2 WHERE id = 'yyy'

You need to set yyy to the ID of the logged in user

Assuming your user table has primary key id, below code will give u an idea.

<?php
$server = ''; // server where your db is hosted
$username = ''; //username of the database
$password = ''; //password of the database
$dbname = ''; //database name of the server
$id = ''; //id of the user you want to change the intro flag of

$vlink = mysql_connect($server,$username,$password);
$dbselect = mysql_select_db($dbname);

//assuming the user already has a row in the table...
$query = 'SELECT COUNT(*) FROM user where id = "'. $id . '" and intro = 1';

$result = mysql_query($query,$vlink);

if(mysql_num_rows($result)>0)
    echo 'User has already been through intro';
else
{
    $query = 'UPDATE users SET intro = 2 WHERE id = ' . $id;

    $result = mysql_query($query,$vlink);
    if($result)
        echo 'Success';
    else
        echo 'Failed...!';
}
?>

Something like this?

$id = (int) $id;
mysql_query("update users set intro = 2 where users.userid = $id;");

It's probably worth running through some MySQL tutorials to give you some of the basics.