Протупил
if(isset($_POST['submit'])
AND $user_id == 1){
mysql_query('UPDATE `mp_player` SET `sostav` = '.$_POST['sostav']);
}
if($user_id == 1) {
echo '<form action = " " method = "post">
<select name = "sostav">
<option value = "0">Зап</option>
<option value = "1">Осн</option>
</select>
<input type = "submit" name = "submit" value = "Изменить">
</form>';
}