There isn't a way to edit a scheduled email-- you would need to delete the scheduled email, and compose it again.
To do this:
- Go to Admin > Messaging.
- Click Scheduled Emails.
- To the right of the email, click the Red X.
- Click the Compose Email button.
- Find members by entering in the criteria desired.
- When finished, click Find Members.
- Fill out the Compose a message form as desired.
- Make sure that you check Schedule email to be sent later, and enter the date/time you want the email to be sent.
- When finished, click Send.