Click here if you are looking for the steps to migrate from "full sync" to (the more reliable) "simple sync" method.
Keeping your existing transaction history is important. If you already use a different app, you probably want to migrate it over into Actual.
Right now, only YNAB4 is officially supported. However, the API allows anyone to write a custom importer. We will work with the community to help write other importers soon.
Migration from the old Desktop App
Are you coming from the original, managed Actual subscription service? That used an older version of the desktop app which doesn't have the export button, but you can log in to the Web version at app.actualbudget.com to export your data in a format suitable for importing in your new instance. See Backups and Restoring.
Instead of creating an export from the Web app, you can also create a zip archive of your budget folder, which is equivalent. The folder is located at
~/Documents/Actual/My-Budget-abc123 by default on MacOS and Linux. The resulting zip file can be imported into Actual via the Web app.
Migration from other apps
There are guides here for some advanced migration tasks.