You can rename the headers of the XLS, CSV, etc. export results file by changing your export profile Field Mapping settings.
1. Open your Document Definition and go to Document Definition -> Export Settings.
2. Choose your Export Profile and click "Edit...".
3. Click "Options..." near the Data format field.
4. Click "Set Up Field Mapping...".
5. Choose the Field you need to be renamed and change the value in the Export fields column.
With the example settings below, the field with the name Main will be exported as Number: