You can update an existing screen in your project by name or corresponding artboard in your design file.
Update a screen by name
To update a screen in your project with an artboard that has the same name in your design file, select the “Name” option under the settings button on the export panel.
Update a screen by the corresponding artboard
To update a screen in your project with the same artboard in your design file, even if they have different names, select the “Corresponding Artboard in <design tool>” option under the settings button on the export panel.
Zeplin will add it as a new version of that screen, without losing your notes. When viewing that screen, members of that project will be able to toggle between the different versions and you can even add a commit message to describe what has changed.