Select a conversation with the message you want to reschedule.
If you don’t see your message, scroll down to the bottom of the conversation. Scheduled messages may appear below sent messages.
Click Edit in the conversation (above your scheduled message), then do any of the following:
Reschedule it: Choose Edit Time, choose a new time, then click away from the schedule to commit the new time.
Send it immediately: Choose Send Message.
You can edit a scheduled message until its delivery time.
Go to the Messages app to revert. See Send a message on MacDelete messages and conversations in Messages on Mac