You can access conversations that were previously archived and return them to the conversations view (iOS app only). 

  1. Tap the Settings icon in the upper toolbar (top left).
  2. Select Archived Conversations.
  3. Swipe left on the desired conversation.
  4. Tap Restore.

Note that a conversation will automatically restore to the inbox the next time either party interacts with it. Manually restoring archived conversations is not available from the Android app.