The first step is to determine the cause of the swelling and hardness. Swelling and hardness in the area of the navel could indicate a hernia with loops of intestine in the swelling. A localized abscess could cause swelling and hardness. Sometimes both conditions (hernia and abscess) exist together. It may not be necessary to do anything. However, sometimes abscesses need to be opened and drained, and if it is a hernia, it may require surgery. A veterinary examination could determine the best course of action.
If this is an abscess, you need to look at your calving area to determine why this calf developed an infected navel. It may be necessary to move to a cleaner calving area or clean the existing area. Dipping navels at birth with a disinfectant or using navel clamps are also options. Hernias sometime occur after an abscess has formed at the navel.