If you have high cortisol, it means that your adrenals are strong and can pump out tons of cortisol.
So the goal is to down-regulate your HPA axis, so your pituitary gland tells your adrenals to stop making so much. Adaptogens are great for this.
Also, a high-carb diet will increase insulin and lower cortisol. (Not junk, but a high-starch diet with whole grains, tubers, vegetables, beans, etc.)
I don't know too much about it, since I don't have high cortisol, but 90% of the information out there regarding AF is actually geared towards high cortisol. And 90% of the adrenal supplements available are for high cortisol.