Push gently from the inside with a round wooden stick, and the sunken place can be pushed out. But don't be too strong, otherwise it will be uneven and ugly. It's best to let people with maintenance history at home do it, and they should have experience.
In order to verify that hot water pouring can really repair car dents without leaving a trace, we first find a bumper with obvious dents, which can't be pushed out directly from the inside. Then we boiled a pot of hot water and poured it directly into the sunken place, half of it, and there was no response at all. After pouring a pot of hot water, the dent did not recover, but it did feel much softer than other places. We pushed from the inside out with our hands, and the sunken place bounced up.