Log in to your Google Merchant Center account
Navigate to the “Products” menu on the left
Select “Feeds”
Click the name of the feed container you want to download
On the feed page, beneath the overview of your feed, click on the blue text that says “Download file.”
What if I see multiple feeds?
You can look at the "products" column showing the product count, "feed name", or "Country of sale" to get clues as to which feed belongs where.
What if I don't see a product feed?
Navigate to the "Products" menu on the left, and select "All Products". This will generate a list of all the active products in your Google Merchant Center, which you can then download by clicking the arrow in the top right-hand corner.
What if I'm using Google Sheets to submit products?
You can download this by navigating to the "Products" menu on the left, select "Feeds", and then "Open" the Google Sheets feed. From there, you will select File > Download > Comma-separated values (.csv).
See here for information from Google:
"Downloaded product data will reflect raw data submitted to Google via your primary feed, in addition to any modifications from supplemental feeds, feed rules, and automatic item updates. This allows you to see your product data as Google uses it, and identify changes you can make to improve your data’s performance."