My underarms are dark how can i lighten them or fix this?

Need to see a doctor: Darkening of the armpits skin can be due to a hormonal imbalance, so a visit to a primary care doctor or an endocrinologist is needed to evaluate and perhaps check some hormone levels. There may be a relationship to pre-diabetes. Sometimes, there is no disorder, but the darkening is instead runs in the family. If it is familial, it will be very difficult to change the coloring.