You can install this extension in any of the normal ways.
It's not yet listed for installation from within CiviCRM.
Download the latest release and unzip it into your extensions directory.
Then navigate to CiviCRM » Administer » System Settings » Extensions and click Install.
Developers can clone the Git repo into your extensions directory, grab the links from https://lab.civicrm.org/extensions/mailchimpsync
In the normal way.