title: Replace a character at a certain index in NSString
body: <p>I'm looking for a way to replace a character at a certain index of a NSString. </p>
  
  <p>For example: <code>myString = @"******";</code></p>
  
  <p>I want to replace the 3rd " * " with an "A", so that the string looks like this: <code>myString = @"**A***";</code></p>
  
  <p>How can I achieve this?</p>
  
  <p>Thanks in advance</p>