You have to do it note-by-note, so you’ll need to go to the Browse window for that.
The feature is described in the link I posted above – but for more detail:
(Make sure all of your devices are in sync first.) Select the deck (in the left sidebar), then Select all of the cards/notes in the table, and go to Notes > Change Note Type – choose Basic (and reversed card). It will automatically make a new reversed card to go with each of your notes.
True, that is another note type you could choose, but I don’t think that’s right one for your case, because you’d have to do an extra step on every note to get it to create the new card. That type is better suited for a situation where you only want a few select notes to make reverse cards, not when you want them for everything. (If you find afterward that there are a few reverse cards you don’t need, it’s easy enough to suspend them when you come across them.)