To overturn something is to either flip it upside down or to invalidate it. An amateur kayaker might overturn her boat, while a judge may decide to overturn a previous judgment.
When you overturn a wheelbarrow or a tricycle, you literally turn it over. When judges overturn earlier rulings or voters overturn laws, they make them invalid, essentially reversing them. An earlier, more intense metaphorical meaning of this word was "violently overthrow a ruler or rulers." Overturn was originally used in the 13th century to describe a turning wheel.