Updating Drush with Composer I've been using Drupal for several years, but only just dipping a toe in with Drush - I've bought an ebook tutorial and working my way through. I've installed Composer, and used it to install Drush. The tutorial book specified a particular build of Drush to install (7.0.0-alpha5). I've navigated to a drupal folder and run 

drush st 


to get installation info which works fine.
The tutorial explains that you don't have to navigate into the drupal folder to do this, instead using

drush --root=PATH-TO-DRUPAL-SITE st


however this brings an error: 

Fatal error: Call to undefined function conf_path()


Googling this error suggests this a problem with the version of Drush and to upgrade.

SO I've navigated into the .composer/vendor/drush/drush folder and run 
    composer global update
but composer says nothing to update, confirming the version as 7.0.0-alpha5 - I assume this is not the latest 7.x version, so do I need something changing in the composer.json file?
If you want to update your global installations of any package under composer, you can just do this:

composer global update

You don't need to navigate to .composer/vendor/drush/drush

However: There are a couple of things:

1) Your global composer.json file might have a version constraint.

Check out drush's packagist page: https://packagist.org/packages/drush/drush

That will show you the available release versions of drush.

Edit ~/.composer/composer.json so that the version constraint for drush/drush is higher than the one you have. Probably something like "~8".

Do composer global update.

2) It's best practice to install drush per project.

The reasons why are a little complex and might not answer the question here, so I'll just link to an article: https://pantheon.io/blog/composer-vs-drush-make-which-should-you-use