Treating underarm hyperpigmentation is tricky because the underarm skin is delicate. However, it is possible to brighten and treat underarm hyperpigmentation with gentle treatments and patience. A few tips that might help lighten underarm hyperpigmentation: (1) Use gentle exfoliants. (2) Never shave without shaving cream. (3) Consider professional micro-needling treatments. (4) Use hyperpigmentation serums. (5) Be patient.