Why not just the following?

str.replace(str.charAt(0), "h")

String.prototype.replaceAt = function(index, replacement) {
  return this.replace(this.charAt(index), replacement)
}

Or, in a slightly less potentially disastrous way of monkey patching…

Object.defineProperty(String.prototype, "replaceAt", {
  value: function(index, replacement) {
    return this.replace(this.charAt(index), replacement)
  }
})