To update a Composer package to the latest version:

composer update <package>

For example, to update phpunit/phpunit to the latest version:

composer update phpunit/phpunit

To show the package version:

composer show phpunit/phpunit

To install phpunit/phpunit version 8.5:

composer require phpunit/phpunit:8.5

To install the new dependency without updating composer.lock, pass option --no-update:

composer require phpunit/phpunit:8.5 --no-update

To update all dependencies to the latest version according to composer.json:

composer update

This also updates composer.lock.

Rerun the command with debug verbosity to see more output:

composer update -vvv

Composer can take a while since it downloads entire package lists.

To see more information about a Composer command:

composer --help

composer update --help

composer require --help