If you navigate away from the page during the export, the CSV file will appear as a previous download in the Gradebook export drop-down menu so it can be downloaded again. The Gradebook will show the date and time you requested the export.
If you made changes to the Gradebook after exporting a file and want to export a new file, click the CSV File link again.
Note: Gradebook export files include the date and time of the download to differentiate between multiple exports. The export filename format is YYYY-MM-DDTHHMM, followed by the course name.