Once you schedule a message to be sent, it is no longer possible to edit the message content or the message details. However, you can remove the scheduled send date and time to re-enter the editing mode.
Click "Update Schedule" in the bottom-right corner of the message screen, then use the "delete" icon in the date field to remove the date and time the message was scheduled to send.
Click "Update" to cancel the send. The editing functions on the message will be restored.