If ↑ ↑ ↑ doesn't work.
If you right click on an editable format (EPUB, AZW3, KEPUB (I think)) in Book details you'll get an Edit book option.
OR
With calibre (library manager, book editor, and book viewer) not running, go to the calibre configuration folder and open the tweak_book_gui.json file in a text editor and change the line that reads :
Code:
"choose_tweak_fmt": false,
to
Code:
"choose_tweak_fmt": true,
Save and close the file and you should be good to go.
BR