When you hold your breath to a certain extent, you can release it slowly, which can prolong the time. One is persistence. You hold your breath for 20 seconds at a time, take a break 10 seconds, hold your breath for 20 seconds, repeat. You will gradually find that your vital capacity has increased. Another way is what you saw when you watched Let Me Be a Supermodel. When your head is above the water, take a deep breath and empty all the waste gas in your lungs. Then take a deep breath, which can last longer than usual.