Composer Self Update Error

I follow the instructions from Elgg:


" Upload Elgg With Composer (recommended if comfortable with CLI): "
Then I got this error.
 " mysite@web10:~$ composer self-update
Updating to version 2.0.8 (stable channel).
   Downloading (100%) 
  [ErrorException]
  rename(/usr/local/php5.6/bin/composer-orig.php): failed to open stream: Permission denied.
self-update [-r|--rollback] [--clean-backups] [--no-progress] [--update-keys] [--stable] [--preview] [--snapshot] [--set-channel-only] [--] [<version>]
mysite@web10:~$ "


What could it be?