Question:

After scheduling an Approved email to be sent, how does a user modify the date or time at which it is to be sent?

Answer:

The Sent Email record Scheduled_Send_Datetime_vod can be modified from the front end-user interface if both the following are true:

• The Sent Email page layout has the Edit button
• Sent Datetime field on the appropriate page layout

An Admin can directly modify the record to update the Scheduled_Send_Datetime_vod from the backend as well. 

Related Documentation:

CRM Help Document: Composing Approved Emails