What is the fastest way to update the whole document (all fields) in MongoDB?

The fastest way is to use replaceOne() in MongoDB. Let us create a collection with documents −

> db.demo431.insertOne({"Name":"Chris","Age":32});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e770ba6bbc41e36cc3cae89")
}
> db.demo431.insertOne({"Name":"David","Age":31});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e770bacbbc41e36cc3cae8a")
}
> db.demo431.insertOne({"Name":"John","Age":24});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e770bb3bbc41e36cc3cae8b")
}
> db.demo431.insertOne({"Name":"Bob","Age":22});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e770bb8bbc41e36cc3cae8c")
}

Display all documents from a collection with the help of find() method −

> db.demo431.find();

This will produce the following output −

{ "_id" : ObjectId("5e770ba6bbc41e36cc3cae89"), "Name" : "Chris", "Age" : 32 }
{ "_id" : ObjectId("5e770bacbbc41e36cc3cae8a"), "Name" : "David", "Age" : 31 }
{ "_id" : ObjectId("5e770bb3bbc41e36cc3cae8b"), "Name" : "John", "Age" : 24 }
{ "_id" : ObjectId("5e770bb8bbc41e36cc3cae8c"), "Name" : "Bob", "Age" : 22 }

Following is the query to update the whole document (all fields) in MongoDB −

> db.demo431.replaceOne(
...    { "Name" : "John" },
...    { "Name" : "Robert", "Age" :45 }
... );
{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }

Display all documents from a collection with the help of find() method −

> db.demo431.find();

This will produce the following output −

{ "_id" : ObjectId("5e770ba6bbc41e36cc3cae89"), "Name" : "Chris", "Age" : 32 }
{ "_id" : ObjectId("5e770bacbbc41e36cc3cae8a"), "Name" : "David", "Age" : 31 }
{ "_id" : ObjectId("5e770bb3bbc41e36cc3cae8b"), "Name" : "Robert", "Age" : 45 }
{ "_id" : ObjectId("5e770bb8bbc41e36cc3cae8c"), "Name" : "Bob", "Age" : 22 }