To update extensions (modules) just run
composer update
See also the solution of possible error: «Target already exists (set extra.magento-force to override)»
If you are developer and you need version control support for installed extensions please use --prefer-source option:
composer update --prefer-source
If an extension installed from a Git repository, then --prefer-source option will create .git subfolder inside the extension folder, so you will get version control support.