You can delete your sent messages by pressing and holding the message for a few seconds and select 'Delete'. You can do this within 48 hours of sending it. This will delete the message for you and the receiver.
NOTE: For safeguarding reasons, you cannot delete a whole chat, only archive it.
To archive whole chats press the 'Ii inside the chat, then 'Archive chat'. This will only archive the chat for you; the receiver will still see the chat in their inbox and will not know that you have archived it.
To re-open the chat, start creating a new message by selecting the receiver/s and the previous content will reappear. The chat is still archived at this point, it's only unarchived if someone sends a new message.
You can also leave group chats by pressing the 'i' inside the chat then 'Leave chat'. The other members of the group chat will be notified that you have left.