List of mirrors: https://github.com/Webysther/packagist-mirror

Change repository in composer.json

"repositories": [
    {
        "type": "composer",
        "url": "https://packagist.fi"
    },
    { "packagist": false }
]

Change mirror in global composer configuration:

composer config -g repos.packagist composer https://packagist.fi

Restore official mirror in global composer configuration:

composer config -g repos.packagist composer https:/repo.packagist.org

List of composer config:

composer config -g -l