There must be something driving the processor to make your phone that hot. The sure fix would be to perform a factory reset to alleviate the problem. You might prefer to go into your settings and see what is running all of the time instead. Finding the culprit and stopping it's process would be a temporary fix. Removing and re-installing the app will likely fix your heat problem.
Be advised that doing a factory reset will return your phone to factory fresh and you will lose accumulated data and settings. It is the sure fix unless it is hardware related which isn't likely.