Instead of clicking "Send," click the dropdown arrow next to the Send button. There are pre-set times you can choose from, or select Choose the date and time to send. It must be at least a couple minutes in the future.
Here's what it looks like in action:
You can cancel the message by going to your newly created "Scheduled" folder on the left:
Either click the Delete icon (garbage can), or open the message and click "Cancel Send":
At this time, there doesn't appear to be a way to edit a scheduled message, but if you provide Google feedback, they may add that feature in the future!