Go to the ‘Search’ tab;
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Search for the work you want to edit;

Select the work you wish to amend;

Click on Additional information;

Fill out the ISWC field with the number that you want to update (dashes are optional);

Fill out the Publisher or Songwriter information (see the image below);

Once you have inserted all the valuable information do not forget to press Add Info first and Save later on the bottom right of the page to save your changes.