Click on any of the Reviewer’s tab you intend to reply
After you selected the desired Reviewer's Tab (How Can I see the Reviewer's Comments?) follow the steps below:
Go to the comment you want to reply to and click Add comment
Enter your reply and Save it
Repeat this for each comment
When you have finished replying to all comments, you can:
Click Submit all comments to notify your comments to the reviewers and the handling editor if you do not have to upload revised files. Note that this will make your replies non-editable.
Click Resubmit Manuscript and proceed with the resubmission. Your comments will be posted and become non-editable once you start the resubmission process. After you upload your revised files and conclude the resubmission, a notification will be sent to the reviewers and the handling editor. Note that you cannot save and resume your re-submission.