Black Baseball Cap

So I have this baseball cap that I bought recently and it's a bit dirty inside the back (the lining is black so as not to show --- it only shows the white part at the rear) ... anyway, I do not machine wash or anything ... Is there a simple way to remove this stain? As with the vinegar and a toothbrush or something?

Be like me. While taking a bath I clean my caps with a shampoo and finger / nail brush. rinse, and hang the night on the handle of the bathtub faucet. In the morning, the ceiling will be dry and ready for use.