When exporting reports to CSV, specify column names into the Export Name field (located at the bottom of the right side panel):
If nothing is entered into this field, the column header will either match the mapped field (if possible) or assigned a number (example: "field 10").
Once export fields are entered into a template, they are saved for next time the template is used.
Changing an Export Name
If your export date range includes reports using the template before and after the export name changed, the .csv file will have two distinct columns and header names. One column will be for the reports before the export name change and the other column will be for the reports created after the export name was changed.
Similar behavior applies if a new data field (with export name) was added to the template.