Calibre's own
Tweak Book feature can also be used for both epub and azw3 books. Just highlight the book in the calibre library window and press T. Then
- Explode the epub
- Open the relevant html file(s) in your simple text editor of choice (Notepad++ is very good for Windows users as you can easily search/replace across all open files). Find and correct the typo(s) and save.
- (optional) Preview the epub to see your changes in the calibre Viewer.
- Rebuild the epub. All done.