Hello and welcome to HCM Ringworm infection generally turns from red to dark colour and then fades away. Do not scratch over the area and complete the treatment course as advised to prevent it coming back. The duration taken for scar to fade depends on your skin. Do you normally have a lot of scars from wounds? or do they clear easily? Do you tan easily. In any case, the area will take a month or two to fade away to normal. You can use gentle lightening creams like kojic acid at night. Nothing too strong to be used in the armpit area. Skin lightening deodorant can be used. Use a lightening soap like dermadew lite. You can also consult a dermatologist regarding procedures like chemical peeling, lasers etc. But this must be one only when infection is completely cleared and skin is back to normal. Also keep a note if you have sensitive skin, or allergies. It will take a while to go. But keep in mind the above points to help quicker improvement.