To download a copy of your data feed from Google Merchant, please follow the tutorial below. This file can be used to alter the data and resubmitted as a supplement feed.
Download Data Feed
Step 1 : Login to Google Merchant Center
Step 2 : Click on Products > All Products in the left hand menu
Step 3 : Click on the download icon
Step 4 : Open the zip file and extract the file.
Step 5: Change the file name extension from .tsv to .csv and you can open it in your spreadsheet editor.