Brown spots on mint
While it is possible that your mint plants are suffering from downy mildew, that cannot be confirmed. If that is the correct diagnosis, the plant will continue to fail. In the meantime, since this is a plant that you intend to use culinarily, we recommend that you simply remove the affected leaves whenever you see them and salvage the good foliage. Consuming a fungal disease will not harm you, but it is ugly and unsavory.