CraftCMS 3 - Composer Update How can I update all the packages under craft/app/vendor for Craft CMS 3 (Yii 2)?
You can't.

The composer.json file (and the composer.lock file so you can't even reverse engineer composer.json) isn't shipped with the official release on purpose.

Craft 3 is still in development preview status (not even alpha release), but the next release will include updated Composer dependencies.