Visit this URL;
Click on the gear icon and select “Settings” from the menu;
Click the “Manage Apps” on left menu and scroll down to WhatsApp Messenger app. Click “OPTIONS” next to it and select “Disconnect from Drive” or “Delete Backup”;
And finally, you can use file explorer app on your own phone to locate the local backup copy of Whatsapp and delete it via this path;
Launch your File Manager.
Tap the WhatsApp folder, a list of all WhatsApp sub-folders will appear.
Tap and hold the Databases file.