Both functions to export a copy of your feed as a CSV file can be found in the Feed Tool.
Export From Modify Products
From Feed Tool > Modify Products, scroll down the the Preview table and locate the Export icon in the top right corner of the table:
Clicking this will prompt and Export Data, where you may choose to export All Columns or only visible columns from the table.
Note that if you have any modifications set up, the export will only include the modified attribute and not the "before and after".
Export From In Merchant Center (Google Only)
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.