Please, read this post first: https://community.sdl.com/product-groups/sdl-tridion-dx/tridion-docs/f/forum/29947/td14-content-importer-replacing-batch-import for an overview.

Starting points for this scenario:

1. All items do already exist in the repository
2. The user running the import has the required credentials to update the items in the repository
3. All files have the correct filenaming convention
4. The files have been moved/copied into a single folder

Possible use cases for this scenario:

1. Illustration place holders are created by authors and exported for offline editing by illustrators. After editing the edited items need to be imported in the repository.
2. A publication is exported via the publish report export option for an offline find/replace operation. The updated items need to be imported back into the repository.

Actually this scenario works more easily than expected (personal note).

The procedure I verified a couple of times:

1. Start Content Importer
2. Select 'Without conversion'
3. Select the folder that contains the to be imported files for the Folder location field.
4. Select a Target folder in the repository. Actually this can be any folder but recommended to select a newly created empty folder of type (None).
5. Click Import and wait for completion

If all starting points are met the items have been updated in the repository.

This also works for a mix of object types (map, topic, libary topic, image, other).

If there is a need to update some files and re-import then this can be done directly in the folder. I have not tested extensively how this updating is supported. To re-import everything it is needed to delete the 'Import Report.log' and 'import.status' files.

See also the tip to remove the Content Imports history to re-start the import from scratch.