If you'd like to edit or cancel a scheduled email, simply locate the message in the your Drafts folder, open the email, click the arrow next to "Send on {date}," then select a new date, time, or "unschedule" to remove it entirely.

A full list of your upcoming Send Laters will be shown in the "Scheduled" tab of the Activity Feed in Gmail. You can also find the scheduled emails in your Drafts folder as mentioned.