yii2 composer update fatal error

1) Remove old version of Composer Asset Plugin:

composer global remove "fxp/composer-asset-plugin"

2) Install newer version. Recommended version for installation is now (by the moment of writing this) 1.1.1 (see official docs).

composer global require "fxp/composer-asset-plugin:~1.1.1"

I'd even recommend to use:

composer global require "fxp/composer-asset-plugin:*"

3) Run composer install in your project folder. In case of errors, delete vendor folder contents and composer.lock file and run composer install again.

If arogachev's answer doesn't exactly work for you, here is a quick fix.

1. Manually delete the vendor folder in your global composer installation. The folder to delete for eg. on ubuntu : /home/user/.composer/vendor
2. Install the newer version of the plugin

composer global require "fxp/composer-asset-plugin:*"

That should be all. You will get all your files back and you can continue