Do I have to update the feed manually every time in the Google Shopping Feed app by AdNabu for Shopify?

How to use the Update Feed button in Google Shopping Feed app by AdNabu

No, you need not update your feed manually. AdNabu automatically updates your feed and pushes the changes made to your feed every 24 hours. 

In case you want the changes to be pushed immediately, you can manually press the Update feed button. The changes made will be instantly pushed to your Merchant Center.