PHP Number guessing program - php

Question

The program is working fine except for the number changing command.
If someone could spot my error that would be greatly appreciated.
The change form comes up but the submit button does not POST to the variable.
change.php
<form class="" action="index.php" method="post">
<p>Min:<input type="number" name="min" value=""></p>
<p>Max:<input type="number" name="max" value=""></P>
<input type="submit" name="valuechange" value="Change">
</form>
index.php
<?php session_start();
?>
<center>
<?php
$minn = 1;
$maxx = 3;
if (isset($_GET["change"])) {
if (isset($_POST["valuechange"])) {
$minn = $_POST["min"];
$maxx = $_POST["max"];
}else {
include("change.php");
}
}
echo "<h1>Guess a number between ". $minn ." and ". $maxx . "</h1>";
if (!isset($_SESSION["number"])) {
$_SESSION["number"] = mt_rand($minn,$maxx);
$_SESSION["counter"] = 0;
}
if (isset($_POST["guess"])) {
if ($_POST['guess']== $_SESSION['number']) {
echo "<br><br><br><br>Correct<br>"."Your number was ". $_SESSION["number"]."<br>"."<a href='reset.php'>Play Again </a>";
session_destroy();
echo "<br><a href='index.php?change=true'>Change Numbers</a>";
}else {
$_SESSION["counter"] = $_SESSION["counter"] + 1;
include("form.php");
echo $_SESSION["counter"]. " Guesses<br>";
if ($_POST["guess"] > $_SESSION["number"]) {
echo "Incorrect your number is too big try a smaller number.";
} else if ($_POST["guess"] < $_SESSION["number"]) {
echo "Incorrect your number is too small try a larger number.";
}
echo "<br><a href='reset.php'>RESET </a>";
}
}else {
include("form.php");
}
?>
</center>