In EndNote online the output styles are not editable by a user.
If you are linked to a University subscription for EndNote online:
The local EndNote online Administrators have the ability to update Styles for their site if EndNote online access is provided by Web of Science. If you have EndNote online access through your institution, you can contact your local Administrator to update existing Styles or upload new Styles.
Your EndNote online Administrator should be able to upload the output style following the instructions below,