There are several ways to cancel and/or reschedule an email, we've outlined these options below:
1) Cancel from your Dashboard
From your Dashboard, select the Scheduled Emails
You will then see a list of all Scheduled Emails. Just click on the Email Subject of the email you would like to edit and a pop-up window will appear.
2) Cancel from an individual Contact Record
Within the Activity Feed tab of a contact record, filter to "show details..." and then select Emails.

Click on the Email Subject of the email you would like to edit and a pop-up window will appear.
From here you can select the Actions drop-down and choose to reschedule or cancel the email.