No. Messages can't be edited or deleted once sent.
Participants also receive an SMS notification the moment you send a message, so assume anything you send has been seen.
Sent a typo or the wrong details? Just send a quick follow-up message with the correction, for example: "Please ignore the time in my last message. Your session is at 2pm." If you've accidentally sent something sensitive, message us on live chat and we'll advise.