The 'Write metadata to files' function of the organizer ONLY adds tags, it does not delete or update the tags in the metadata section.
To edit one picture file directly in the metadata section independently from what is registered in the catalog, use the 'Information' panel on the right of the organizer. Be sure to be in the detailed information mode (the icon with 3 horizontal bars on the right of the 'metadata' title.
Click on the 3 dots icon just right of the IPTC title.
That displays the details of the metadata. There you can edit your tags as you want. Note that this is direct access to the file and does not change what is in the catalog database.
If you want to edit many files at the same time, you select your files and follow the above method.
You click on the button to modify IPTC data.
There you can clear all tags in the files. Afterwards you simply issue a 'write metadata to files' to update the files from the contents of the catalog.
There is no way to edit the catalog from the files themselves. For instance, if a file is edited in another catalog, the added or modified items won't be updated in the current catalog. The only way is to remove from catalog and re-import the file.