Hi there -
You'll need to change the note type in use for the notes you want to have reverse cards, such that they have a second card in the note type where the Front / Back fields are switched -- https://ankidroid.org/#reverseCards
You can change note type note by note in AnkiDroid, in a batch of
multiple notes at once in Anki Desktop, or you can alter an
existing note type to add a new reversed card type to it, and all
notes using that note type will get the new reverse card