Fill a water kettle and turn it on to high heat. The water should boil within a few minutes. Get out a heat-proof bowl and carefully pour enough hot water to fill the bowl halfway. Avoid pouring in too much water or it will spill over the side of the bowl when you add the Nutella. Remove the lid from the jar of Nutella and set the jar into the bowl of hot water. Ensure that water can't get into the jar. If you want to heat a small amount, you can scoop some into a small bowl. You can place this bowl of Nutella into the bowl of hot water as long as water won't come over the sides and into the Nutella. Let the Nutella sit in the hot water for about 5 minutes. Stir the Nutella with a wooden or metal spoon and remove it the jar from the water if it's melted. If not, let the Nutella sit in the hot water until it's melted. This is a gentle way of heating the Nutella because you won't actually cook or overheat it.

Summary:
Bring a pot of water to boil. Pour boiling water into a bowl. Place the jar of Nutella in the hot water. Soften and stir the Nutella.