Google: How to Update Your Google Product Feed Manually
If you've made updates and you'd like them to be immediately reflected in your Google Merchant center, you have the option to regenerate your feed and manually upload it to Google at anytime.
This will require that you login to your MightyMerchant Site Manager (to access the feed) and your Google Merchant Center (to upload or automate its submission).
To access your feed management area in MightyMerchant, login to your Site Manager as usual and click the " Data Feeds" link in the navigation area.
To manually upload your feed, first download the latest feed from your site by clicking the download link in the "Data Feeds" area of your Site Manager, and save the file to your hard drive.
Once you've done this, follow Google's instructions here to upload it to your Google Merchant Center: