Im not sure what your body type is, but the only two options are surgery or to lose weight. If you are already at a healthy weight and the c cup size is bothering you, it might be time to see a surgeon.
Try using a minimizer bra. Of course you will still be a 'c' cup, but you'll look like a ' b' cup. I agree with other answers, if you want it to be permanent, try weight loss first, then last option would be breast reduction surgery.
only way you can into a B bra size is to lose weight. don't eat any sweets and only eat one plate at lunch and supper. eat very light for breakfast like yogurt or fruit. don't forget to exercise every day.