When scheduling a Follow-Up Email, you have the option to choose how long to wait to send the email. If you want to send your follow-up at the same time as the original email, just a few days later, use the plus and minus buttons to add or remove days.
If you want to specify a different send time, click Custom timing. Date and time pickers will appear. Select the date and time to send your follow-up.
Don't forget to enter a new subject line and click Schedule.
For more information on sending follow-up emails, checkout this article.