Note: You won’t be able to edit a sent message after five minutes.
To edit a message that was sent very recently. Go to the message-right-click-menu as directed by the arrow from the figure below.
Select Edit as directed by the arrow from the figure below.
Make the necessary edits on the sent message and re-send it.
You can find the edited message with a pen marker along with the edited time shown within the re-sent message.