How does MongoDB Update() method work to set records of entire field?

You can use $set operator along with update(). Let us first create a collection with documents −

> db.workingOfUpdateMethod.insertOne({"ClientCountryName" : "AUS"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cd506fe2cba06f46efe9efa")
}
> db.workingOfUpdateMethod.insertOne({"ClientCountryName" : "AUS"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cd507022cba06f46efe9efb")
}
> db.workingOfUpdateMethod.insertOne({"ClientCountryName" : "AUS"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cd507022cba06f46efe9efc")
}
> db.workingOfUpdateMethod.insertOne({"ClientCountryName" : "AUS"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cd507032cba06f46efe9efd")
}

Following is the query to display all documents from a collection with the help of find() method −

> db.workingOfUpdateMethod.find().pretty();

This will produce the following output −

{ "_id" : ObjectId("5cd506fe2cba06f46efe9efa"), "ClientCountryName" : "AUS" }
{ "_id" : ObjectId("5cd507022cba06f46efe9efb"), "ClientCountryName" : "AUS" }
{ "_id" : ObjectId("5cd507022cba06f46efe9efc"), "ClientCountryName" : "AUS" }
{ "_id" : ObjectId("5cd507032cba06f46efe9efd"), "ClientCountryName" : "AUS" }

Following is the query to update and set an entire field. Here, we are updating the field “ClientCountryName” −

> db.workingOfUpdateMethod.update(
... {"ClientCountryName" : "AUS"},
... {$set: {"ClientCountryName" : "UK"}},
...  {multi: true});
WriteResult({ "nMatched" : 4, "nUpserted" : 0, "nModified" : 4 })

Let us check the all the documents once again −

> db.workingOfUpdateMethod.find().pretty();

This will produce the following output −

{ "_id" : ObjectId("5cd506fe2cba06f46efe9efa"), "ClientCountryName" : "UK" }
{ "_id" : ObjectId("5cd507022cba06f46efe9efb"), "ClientCountryName" : "UK" }
{ "_id" : ObjectId("5cd507022cba06f46efe9efc"), "ClientCountryName" : "UK" }
{ "_id" : ObjectId("5cd507032cba06f46efe9efd"), "ClientCountryName" : "UK" }