Use a wire brush, there are specialty brushes made for that purpose or you could use sandpaper. You can also use baking powder and water to neutralize the acid.
I know I’m late but use an old toothbrush before a wire brush so you don’t wear down the terminals corrosion protective layer. And also you can use coke or baking soda and mixed with hot water To clean off corrosion and then protect with corrosion control or petroleum (Vaseline). Best way to get it all clean is to remove the battery set in cardboard and soak with the baking powder and water solution which neutralizes the battery acid.
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