Customizing an Email for One Contact in a Sequence

Go to the Sequences page
Click into your Sequence
Click the Emails tab in your Sequence
Click the Filters icon at the top left of the Emails tab and use the Keywords field to search for the contact's name
Once the contact is returned, click on the Scheduled tab to view which emails have not been sent yet (and are therefore still editable)
Click on any email in the list to bring up the template editor
Make your changes
Your email will automatically save any edits after a couple of seconds

The email for that one contact has been updated and will now be sent at the scheduled time.

Edit a Specific Email in a Sequence