
Okay, there can be a few causes of this - one may be relatively simple, other may indicate a skin condition.
Pigmentation change, that is a potential cause is very common, and if this is what it is there is not much you can do about it.
First of all, part of a potential cause could be due to sweat being regularly present in your groin region.
Are you sportingly active, such as do physically active sports training regularly, or things like gym workouts where you sweat a great deal. Or it could be down to the environment in which you are in most days, is this quite warm usually? Do you wear clothing that encourages sweat?
Do you have, or can easily purchase either liquid coconut oil or SHEA butter?
Get either of these and apply a very light coating to the entire shaft of your penis, best time to do this is immediately after a shower when you have fully dried yourself.
Apply for at least every other day for around 3 weeks and report back on what is going on …