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Navigate to Admin > Management > Geography > Postal Codes.
Export the postal codes that are to be updated. You can export a subset by using the search form at the top of the page. In the example below, only postal codes in the state of California are selected.
Edit the exported postal codes with a spreadsheet application and save (retaining the CSV format).
Ingest the edited postal codes file specifying Postal Codes as the ingestion type.
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