How to replace a letter in a string with another letter in ruby : so for instance when i type "elijah".my_method()   in the irb command i want it to return "3lijah" because i want to replace the letter "e" with the number "3"

I've tried using sub and gsub methods along with replace but none of these are allowing me to replace that letter. Here is my code

class String
  define_method(:leet_speak) do
    containsE = self.include?("e") 

    whereIsE = self.index("e")
    whereIsE.replace("3")
  end
end