There is basically no way to specifically 'train' or work-out or stretch to increase the girth of your penis.
Penis size is a function of heredity, and you are most likely to wind up looking a lot like your father did.
Second, your genitals responded to your hormones during puberty, and that converted them to an adult status.
Third, sexual stimulation and arousal make the penis swell to an erect state. But there are limits to how much blood and how large those sacks will fill. Though some days you may feel "fuller" than others, the size is related to your arousal state. And that's in your mind, and in the stimulation.
Fourth, though a lot of people will make claims that you can stretch, tug, pull or misshapen your penis to your will, there is no proof that this works.
Otherwise, we'd all be walking around with huge sausages between or legs, now wouldn't we??!!