Head to your archive to find your archived conversation.

You can do this in your yaps inbox by re-clicking the yaps icon (This allows you to toggle between the yaps inbox and archive)

Once you've found the conversation you'd like to unarchive (you can use the search bar to find the yap or scroll) swipe left on the chat to uncover advanced settings.

Select alert & unarchive to move the yap back into your yaps inbox.