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I am working on a WordPress plugin that will be accessible only to a group of a few selected website owners. I have installed my plugin to those sites already. There is active development going on for this plugin. And we need to upload changes to all the sites. Manually uploading the changes in all sites will be a tedious process. So I would prefer to upload changes to all of those websites at the same time.

How is this possible with WordPress?

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Yes you can. Check this Repo

https://github.com/YahnisElsts/plugin-update-checker

This is a custom update checker library for WordPress plugins and themes. It lets you add automatic update notifications and one-click upgrades to your WP Plugin. All you need to do is put your plugin/theme details in a JSON file, place the file on your server, and pass the URL to the library. The library periodically checks the URL to see if there's a new version available and displays an update notification to the user if necessary.

You can either manage your update/source code on your own server or you can store them on BitBucket or Github

From the users' perspective, it works just like with plugins and themes hosted on WordPress Repo