How can i get rid of an underarm rash?

No antiperspirants: Most common cause is using antiperspirant which irritates and clogged the pores. Could also be yeast infection so check your blood sugar.

Scented deodorants: This can be from excessive sweating creating an environment for candida or from an allergic contact reaction to the deodorant you are using. A topical cortisone cream may help if it is a contact allergy , reducing the sweating with a fragrance-free deodorant +/- an antifungal cream may control the candida infection. See a dermatologist if the above measures failed.