Frontiers review is a collaborative process, where authors and reviewers work together to improve the manuscript. Each reviewer might have comments that give a different insight to the work, hence it is important that all reviewers' comments are addressed before submitting any revisions (How can I post a reply in the Review Forum?).
After you have replied to at least one comment from each 'active' reviewer:
Click Resubmit Manuscript
Go through the Submission screens and make any modifications necessary to the saved information. In particular:
Author Information changes: You can update the name, the email address of an existing author, change the author order, or add/remove co-authors
File upload: If you need to upload a new manuscript or replace files, please do so by using the 'Replace' button on the right side of each file
Replace Manuscript: you will need to replace both the Word MS (or LaTeX) version and the pdf version
When you have finished Uploading/Replacing the files, click on Next until you get to the 'preview' page. Allow for the Frontiers pdf document to be created and then, click Submit
If resubmission has been successful, the following things should appear in the Review Forum:
Updated Article History: A new entry in the History of the manuscript indicating that you have resubmitted;
View All files: A new row with the new files and the resubmission date.