Hi,
On the first instance, why do you feel like increasing size of your penis. You are having penis size more than the mean size of general population. The thing which matters the most "is your partner satisfied or not with your sexual intercourse?". Even if not, penis width or girth matters more than the length for sexual satisfaction. So the most crucial thing to decide further course of action is your partner's satisfaction!
Next Steps
if in doubt, consult a psychiatrist