What should I do in order to shorten my philtrum? (Photo)

Upper lip length (and length of philtrum) should be evaluated in relation to nose, lips and chin. A full face view and full profile view would help to provide more accurate evaluation. From what I can see on your photo based on size of your lips and chin your philtrum  seem to be within normal range. I would not suggest anything done.

You have a minimal difference and it does not make u look masculine. Leave it alone!       

It's hard to tell the true relationship of your philtrum to your overall facial proportions from the photo alone.  One thing to think about may be to plump the upper lip a bit and see if that improves your overall lip and facial balance.  Hope this is helpful.