PHP Insert values into Database at certain position - php

Question

So I'm currently creating a Online Game and already got the login and registering working but now I'm working on submitting Stats such as health and a player's level and retrieving that.
So in this PHP file I'm able to retrieve the info of a certain player but I don't know how to submit new stats into a certain player's row. I'm still learning PHP so please help me out.
<?php
// Database Things =========================================================
$host = "localhost";
$user = "lyth_com_Spillnk";
$password = "INTERESTED?";
$dbname = "lythumn_com_Spillnk";
mysql_connect($host, $user, $password) or die("Can't connect into database");
mysql_select_db($dbname)or die("Can't connect into database");
// =============================================================================
$Act = $_GET["Act"];// what is action, Submit or Retrieve?
$nick = $_GET["User"];
$health = $_GET["Health"];
$level = $_GET["Level"];
$xcood = $_GET["X"];
$ycood = $_GET["Y"];
if($Act == "Retrieve"){
$SQL = "SELECT * FROM Stats WHERE Username = '" . $nick . "'";
$result_id = #mysql_query($SQL) or die("DB ERROR");
$total = mysql_num_rows($result_id);
if($total) {
$datas = #mysql_fetch_array($result_id);
echo ($datas["Health"], $datas["Level"], $datas["X"], $datas["Y"]);
}
}
if($Act == "Submit"){
$SQL = "SELECT * FROM Stats WHERE Username = '" . $nick . "'";
$result_id = #mysql_query($SQL) or die("DB ERROR");
$query = "INSERT INTO Stats (Username, Health, Level, X, Y) VALUES('$nick', '$health', '$level', $'xcood', $'ycood')";
mysql_query($query) or die("ERROR");
mysql_close();
echo "Submitted";
}
// Close mySQL Connection
mysql_close();
?>
It's mostly concercing this piece of code:
if($Act == "Submit"){
$SQL = "SELECT * FROM Stats WHERE Username = '" . $nick . "'";
$result_id = #mysql_query($SQL) or die("DB ERROR");
$query = "INSERT INTO Stats (Username, Health, Level, X, Y) VALUES('$nick', '$health', '$level', $'xcood', $'ycood')";
mysql_query($query) or die("ERROR");
mysql_close();
echo "Submitted";
}
As you are able to see I already retrieve the Index of where the player's stats are located so how do I insert values there?
Thanks in advance!