The `$not` operator in MongoDB inverts the effect of a query expression and returns documents that do not match the query expression. Here’s how to use it:
Firstly, you need to connect to your MongoDB database.
Once you’re connected to your MongoDB database, you can write a query using the `$not` operator like this:
```
db.collection.find( { field: { $not: { $gte: 1.99 } } } );
```
In this example, the `$not` operator retrieves any documents where the `field` value is less than 1.99. The `$gte` operator means “greater than or equal to”, so `$not: { $gte: 1.99 }` seeks for values that are not greater than or equal to 1.99.
Moreover, you can use `$not` operator with regular expressions. For example, it can be used to get documents where field does not begins with a specific word or letter:
```
db.collection.find({
field: {
$not: /^A/
}
});
```
Above query will return documents that doesn’t begins with letter ‘A’.
