On days with extreme heat or high humidity, your heart rate will be significantly higher at your normal running pace because your body is working overtime to cool itself down.
In these conditions, you should always run by 'feel' (Rate of Perceived Exertion) rather than strictly following URUNN's original pace guidance.
If URUNN prescribes a 5:30/km easy run, but it feels like a hard effort due to 30°C (86°F) heat, slow down to whatever pace allows you to breathe easily. You can easily adjust the target pace for that specific day so the audio coach doesn't push you too hard.
Simply tap on the specific run in your calendar and select Adjust Paces. This allows you to lower the pace targets for just that single workout, keeping the rest of your training plan intact. Safety and hydration always come first.