This is now available as of 0.19. Before that you needed a 3rd party tool like lasim.

User data export/import

Users can now export their data (community follows, blocklists, profile settings), and import it again on another instance. This can be used for account migrations and also as a form of backup. The export format is designed to remain unchanged for a long time. You can make regular exports, and if the instance becomes unavailable, register a new account and import the data. This way you can continue using Lemmy seamlessly.