It is hard to give a specific answer to your question, because these spots on your skin could be many different things. Therefore, I would suggest that you go to see your primary care doctor
or a dermatologist
about the issue. They can examine your skin and help figure out what the exact cause of the spots is and what you should do about them.
Since the rash is brown, and not reddened or itchy, it is unlikely to be an allergic rash like contact dermatitis or another type of inflammatory condition of the skin such as eczema. On the other hand, chronic sun damage to the skin as well as normal skin aging can cause the appearance of brownish spots on the skin, called lentigo, which might be what is going on.
If the brown discoloration is lower down below your jaw, in the folds or creases of your neck, then it could be a condition called acanthosis nigricans, which typically is a brownish discoloration of the skin that is associated with the metabolic syndrome (high blood glucose levels, high blood pressure
, and being overweight).
I would start by seeing your doctor as soon as possible so that they can give you an opinion!