void AccountManager::changeUserName(const string& oldName, const string& newName) {
   }

1. Use myAccounts.find() to get an iterator it pointing to the pair containing userName and its password.
2. If it is different from myAccounts.end():
   • Use it->second to retrieve the password, and save it in password.
   • Try
     1. Use erase(it) to remove the (oldUserName, password) pair from myAccounts.
     2. Use createNewAccount(newName, password) to replace it.
     If either throws an exception, throw it on to the sender of this message.
3. Otherwise:
• Throw an exception.