The following is for Android users, not sure about Apple's settings. On Android, go to settings and find the tab labeled Google and click on it. There should be an option to opt into Covid-19 exposure notifications, once that is you download a compatible contact tracing app. You will need Bluetooth and Location to be enabled in order to receive notifications.

Sure would have been nice to have received a heads up from Google that they added such an option, I sure didn't ask for it, and won't be using it being I seldom ever use Bluetooth or Location services. Hope this was helpful to those who were yet unaware of such.