I think you should be thinking more along the lines of something like this:
if (strlen($newpassword) > 25 || strlen($newpassword) < 6)
echo "Password must be betwwen 6 & 25";
if ($newpassword !=="")
$querychange = mysql_query("
UPDATE users SET password='$newpassword' WHERE username='$user'
if (!mysql_query($querychange ))
echo "There was an error in updating your password...";
die("Your pass has benn changed.<a href='index.php'>Return</a> to the main page");
As your application will end if the criteria are not met, there is no need to the added else, you are basically creating an unneccessary complication in that.
You would ideally handle something like this by checking all of the input, then if this is all in order, proceed, first by cleaning the input, then whatever else you have in mind.