How do I set it to automatically update at a certain time ?

For iOS you can update at night:

Update your device wirelessly

If a message says that an update is available, tap Install Now. You can also follow these steps:

1. Plug your device into power and connect to the Internet with Wi-Fi.
2. Tap Settings > General > Software Update.
   
3. Tap Download and Install. If a message asks to temporarily remove apps because iOS needs more space for the update, tap Continue or Cancel. Later, iOS will reinstall apps that it removed. If you tap Cancel, learn what to do next.
4. To update now, tap Install. Or you can tap Later and choose Install Tonight or Remind Me Later. If you tap Install Tonight, just plug your iOS device into power before you go to sleep. Your device will update automatically overnight.

For mac there is no such feature.