composer --version will show the Composer version. If you don't have the update command, it's either very old or is a modified version of Composer.
Is this a managed hosting ? If it's managed, you should ask your host whether they can update Composer for you. If it's managed and they won't update, you might need to download the Composer binary and directly call it with php /path/to/composer [command]. If this is a VPS, just install Composer from scratch again, it should override the existing version.
