Take a bath and dry your underarm completely. Cut a lemon into two pieces and rub the lemon slices in the underarm. Lemon, being a citrus fruit, is a natural astringent and shrinks the pores that cause sweating and odor. You can also rub potato slices and then apply any natural deodorant to your underarms.
With these treatments, never wear tight clothing; prefer to wear loose fitted cotton clothing in summer. If you have strong underarm odors then make sure to clean the area frequently with wet cloth often and then immediately rub the area with dry cloth because if the area remains wet, it will induce growth of bacteria.