How to get rid of lightening highlights and become darker permantly

If you lightened dark hair, there is nothing to remove. You already removed pigment from their hair. That is how bleaching/lightening/lifting works.
You need to add pigment. If you add pigment, its still not permanent. Artificial color fades revealing the lack of pigment. You can keep adding a color that matches your natural color and extend its effect with a color deposit shampoo or conditioner till it grows out enough to get cut off.

Just get a darker dye and dye right over it. Darker will cover up the prior colored hair. Get a permanent ammonia based dye like L'Oreal. I reccommend L'Oreal because they have a pro line we use in salon and if you want a salon color in the future their product works with pro chemicals to be on the safe side.