If you are looking to export your feed for free, there is an option in Add / Remove Products.
The export function can be found by navigating to the In Merchant Center tab. This will export your current feed as it appears in Google Merchant Center. This is only available as a CSV file export:
Locate and click on the small arrow icon:
The export will be downloaded to your computer. With this file you can edit as you see fit, such as by:
Adjusting column headers
Editing product attributes
Filling in data missives
Additionally, a CSV file can be imported to Google Sheets which is a compatible format for Supplemental Feeds in Google Merchant Center.