title: Indexeddb - Update record by index key
body: <p>I have an object store with keypath "id" (autoincrement), and a unique index on that store with keypath "myId" that is generated from a server.
  When I need to update a record, I won't have "id" available, so how can I update the record just by using "myId" considering that the index is unique?</p>
  
  <p>I already tried using mystore.put(record), but that gives an error since a record with "myId" already exists.</p>