There are two ways your follow-up email can be sent, depending on whether you initiated the automatic follow-up feature, or if you sent the initial outreach email manually. Here is a Knowledge Base article explaining the differences between automated and manual outreach.
By initiating the auto follow-up feature, your follow-up attempts will be sent out automatically, according to the days between attempts and your outreach schedule.
If an opportunity responds to your email and you wish to continue outreach to the next contact, you must wait the set days between follow-up attempts before it sends.
If you chose to manually follow-up, your second attempt will appear in your 'Compose' stage (if you haven’t received a reply) after the number of days you set between your follow-up attempts. This number is set while creating your templates on the 'Templates & Email Sequence' screen.
If you’d rather not wait for the next email in the sequence, you can go to the 'Opportunity Details' page for the opportunity, find the contact, and click on their email address, then click 'Send New Email'.