Here's an example where we change the call of a record from kf6gpe-7 to kf6gpe-9 by specifying its ID, and then performing a merge with the new data:

var call = "kf6gpe-7";

db.merge(id, {call: 'kf6gpe-9'}, function(error, doc) {
  db.get(id, function(error, doc) {
    console.log(doc);
  });
});

As you can see from the function, merge takes the ID of the record to merge, and a JavaScript object with the fields to replace or add to the existing object. You can also pass a callback, which is invoked by merge when the operation completes. The error value will be non-zero in the event of an error, and the document is returned as the second argument. Here, we just log the contents of the revised document to the console.