You've likely been saying it wrong your whole life. Although it's not really a "where you're from" language problem, so much as it is a "you're just plain wrong" problem.
In the United States, the language debate can get pretty heated. Besides you guys or y'all, do you say Soda, Pop or Coke? Depending on where you're from, you'll use a different word to describe the same thing.
For example, this is a crayfish:
And this is a map of all the different words people in the United States use to describe the crustacean:
Back to the Reese's argument: there are two pronunciations of the word: Rees-EES and Rees-Is. They aren't specific to regions like the word 'crayfish,' but people to generally speak the same way as those around them.
However, unlike crayfish, there is only one right way to say Reese's.
Have you been pronouncing it wrong this whole time? Find out on the next page!