driver-core/src/main/com/mongodb/client/model/Updates.java
public static Bson rename(final String fieldName, final String newFieldName) {
        notNull("newFieldName", newFieldName);
        return new SimpleUpdate<String>(fieldName, newFieldName, "$rename");
    }
public static Bson rename(final String fieldName, final String newFieldName) {
        notNull("newFieldName", newFieldName);
        return new SimpleUpdate<String>(fieldName, newFieldName, "$rename");
    }
Creates an update that renames a field.

@param fieldName    the non-null field name
@param newFieldName the non-null new field name
@return the update
@mongodb.driver.manual reference/operator/update/rename/ $rename