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If your CSV file has a column in it that maps to the key field of the table then all you have to do is import the CSV file into QuickBase. To learn more, please see our Import Data from Outside QuickBase help topic.
If your CSV file doesn't have a column that maps to the key field in QuickBase, but it does have a column that maps to a unique field in your QuickBase table then you can temporarily or permanently switch the QuickBase key field to the unique field to do your update via import. For more information, please see our Set the Key Field of a Table help topic.