Open the Settings app. You can do this by either tapping Settings within the App Drawer, or tapping the gear icon in the Notification Shade. From within Settings, tap "About phone," and then scroll to ...
To be clear, these methods don't stop robocalls. They just send them directly to voicemail without ever ringing your phone.
The tool began rolling out to users back in August, and now, as part of the November Feature Drop, Google is porting it over ...