Use the website, it has an option to export your data to a file, and then import it. You don't need any 3rd party apps or anything.

Not in the app but here are two tools for migration by Emotet and elvith.

https://stablenarwhal.github.io/Lemmy-Userdata-Migration/

Features:

• Export user data from any Lemmy instance (>=v0.19)
• Download user data as a text file
• Modify user data, e.g. to add or remove followed users/communites (Example in Wiki)
  • "display_name" ​
  • "bio" ​
  • "avatar" ​
  • "banner" ​
  • "matrix_id" ​
  • "bot_account" ​
  • "settings" ​
  • "followed_communities" ​
  • "saved_posts" ​
  • "saved_comments" ​
  • "blocked_communities" ​
  • "blocked_users" ​
  • "blocked_instances"
• Transfer user data to the target account on the target instance

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https://elvith-de.github.io/lemmy-migration/

Features:

• Login and export settings from any Lemmy instance (e.g. feddit.de)
• Optionally: Find local communities on the target instance that match followed communities
• Optionally: Backup your settings to a file (can be imported on any Lemmy instance in your profile)
• Login and import settings to any Lemmy instance (e.g. feddit.org)