Instead, your local health authority will only need to opt-in to the program before you can turn the feature on.
You can download and install the update on your iPhone by going to Settings > General > Software Update and following the prompts.
After the update is installed, you can find the new Exposure Notifications option in the Settings app. Selecting it will provide more info, as well as give you the option to turn on exposure notifications. You'll then be asked to select your country, region or state. If your local health officials have opted into the program, you'll be able to complete set up.
If it's not available in your area yet, you can turn on availability alerts in the Exposure Notifications settings page.