Use data Loader to update a large number of records at one time.
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Obtain your data by performing an export of the objects you wish to update, or by running a report. Make sure your report includes the record ID.
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As a backup measure, save an extra copy of the generated CSV file.
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Open your working file in a CSV editor such as Excel, and update your data.
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Launch Data Loader, then click Update. Note that matching is done according to record ID.
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After the operation, review your success and error log files.
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If you made a mistake, use the backup file to update the records to their previous values.