Thanks @laurent!
For anyone else who wants to do this, the steps are:
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Make a backup of your Joplin data: ~/.config/joplin-desktop.
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In a terminal,
sqlite3 ~/.config/joplin-desktop/database.sqlite
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In sqlite3,
select item_id, body_diff from revisions order by created_time;
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Look at the results to find what you want to restore. Copy the item_id.
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Download Joplin 2.0.8 from Release v2.0.8 · laurent22/joplin · GitHub (I first tried 2.0.4 and it didn't work).
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Open Joplin 2.0.8. This upgrades your Joplin database so the old version can't use it anymore. That's why you need a backup.
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Ctrl+Shift+P (or Command+Shift+P), then "restoreNoteRevision NOTE_ID".
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Save your restored note somewhere outside Joplin.
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Replace ~/.config/joplin-desktop with your backup.
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Open the old Joplin and import your saved note.
Once version 2 is released, you can skip 5-6 and 8-10.