When logged in, click on APPS>ALERTS.
There will be one alert there called Main Frontpage Alert.
Click the title of that alert Main Frontpage Alert.
Make sure that if you want the popup to go live, you make sure it is set to ACTIVE.
Edit the text to what you want. Click Save.
Now on the homepage, you should see the popup box come up. If you see the popup box on EVERY page, please go back to edit the popup and enable ‘on specific url’. Then type in your church domain name on the right. An example is pictured below: