title: How to modify key in hashmap?
body: <p>I am doing an assignment about a flight booking app. I am encountering the problem about modifying the keys of a <code>Hashmap</code> of stored users (including clients and admins). </p>
  
  <p>The keys are a list of strings of personal information like emails, passwords, addresses and ect. What should I do to change some of the information?</p>
  
  <p>The format of personal information is like this: Number, DepartureDateTime, ArrivalDateTime, Airline, Origin, Destination and Price.</p>