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If you are not confident with MySQL queries you can use the Truncate audit trail task in the admin console to do the same thing.
If you would prefer to keep some of the records you can set the parameters of the days you want to keep and run the Purge old audit trail records task,
If you wish to backup whilst purging then you can run the Export/Purge old audit trail records task.
Export and Purge old audit trail records
Please note that Purging is a longer process than truncating, and export/purge will take even longer.