ActivityPub

Well, the official way is manual - open your settings on the old instance and export them. Do the same in your account on your new instance and import them.

One thing to note: you can only migrate your saved posts/comments/..., your followed communities, your settings and profile text and such. Your posts and comments themselves, the amount of up- and downvotes you received will stay on your old instance and you'll start fresh on the new instance on this regard.

Lemmy - as many websites - has two parts. The first is "the website" (or "webapp" or "frontend") that runs in your browser and displays the content. But the heart is the second part in the background. That "the backend" (or API) where you can request data from Lemmy in a machine readable format and where you can send data (e.g. to create posts, comments,...).

Since Lemmy allows you to use it with many different apps and alternative frontends, the backend allows you to query it from basically anywhere. For some features you need to be logged in, but any app can login on your behalf if it knows your password (and 2FA code if applicable).

That's what that little project does. You tell it your old and new instance, give it your credentials for each of them and it will automate the export and import of settings. And since the settings are in a machine readable format and it can use the search on the new instance, it can offer you to check what communities you follow and if there are some local communities on your new instance with the same name and allows you to follow them, too.

The only thing to keep in mind is, that while the code is just a little bit of HTML, CSS and JS and could run directly from your local PC after a download, the way the Lemmy API works forces you to load the tool from

a) a real domain name instead of just your filesystem and b) you need to load it with TLS encrypted (HTTPS)

Both are solved by the GitHub Pages deployment, but no one is stopping you from putting it on your Webspace, Server, ...

That's also the same way all the alternative user interfaces for Lemmy work - e.g. Tesseract which you could host on say tesseract.example.com while still accessing your instance.

Example: My instance uses the default Lemmy user interface on https://feddit.org/ but I can also use Tesseract by visiting https://t.feddit.org/. Or you can use it with Photon: https://p.feddit.org/. Or Alexandrite: https://a.feddit.org/. Or Next: https://next.feddit.org/. Or...