If you have sent a message and you have realised that you have sent it to the wrong person(s), you can do the following:
Go to "Quick Messages" > "Sent" > find that message and click on the 3 dots in the top right corner > click on the "Edit Message" button.
Important: You will ONLY be able to edit messages sent by you, NOT messages sent by other users.
Delete the text and attachments from the message and write another message (e.g. Wrong message), then press the "Save" button.
Note: The moment you press the save button, any of the recipients will open the message, they will only see the new text, not the old one.