In your Merchant Center account, click the Products drop down menu from the left navigation panel. Click Feeds. Locate your feed from the list of registered files. In the "Feed name" column, click the feed name associated with the file you'd like to submit.

Merchant File Upload. Mass merchant upload enables you to board or update multiple merchants using an XML or CSV file. You can also search for uploaded files and view file status reports. Only administrator users can use the merchant file upload feature.

A feed is a file that contains a list of products you want to advertise through Merchant Center. When you add your products, you'll assign attributes to each one. Your product feeds will use these attributes to group your products together.