How to use update operator in MongoDB?

The update operator in MongoDB is used to modify the values of specified fields in documents. The basic syntax of the update operator is as follows:

```
db.collection.update( , , { upsert: , multi: , writeConcern: , collation: , arrayFilters: [ , … ], hint: }
)
```
- `` is the selection criteria for the update. The update method updates the values of the existing document that match the query criteria.

- `` is the modifications to apply.

- The optional parameters like `upsert`, `multi` etc. further control the operations.

For example, if you have a collection called `students` and you wish to update the student document where `name` is `Alex`, you would do the following:

```
db.students.update( { “name” : “Alex” }, { $set: { “grade” : “A” } }
)
```
The `$set` operator replaces the value of a field with the specified value.

`upsert` option:
If set to true, this will create a new document when no document matches the query criteria.

`multi` option:
If set to true, this updates multiple documents that meet the query criteria. If set to false, it updates one document.

Note: As of MongoDB 4.2, the `db.collection.update()` method can update only a single document. For multiple documents updates, use `db.collection.updateMany()`.

In MongoDB 4.2 and earlier, you need to use the `multi` option to update multiple documents.

Documentation: https://docs.mongodb.com/manual/reference/method/db.collection.update/