How to update an object of a collection when update collection of that object

Working with Data Drivers
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1

In .net core I have 3 collection/class

     public class UpperZone : MongoEntity
    {
         public string UpperZoneName{ get; set; }
         public List<Country> CountryList { get; set; }
    }

     public class Country : MongoEntity
    {
        public string CountryName { get; set; }
        public List<Region> RegionList { get; set; }
    }

     public class Region : MongoEntity
    {
        public string RegionName { get; set; }
    }

These collections are kept in mongodb. There are multiple countries within the UpperZone and multiple regions within the Country. When I pull a Region from the Region collection and update its name, the name of the corresponding field in the Country and UpperZone is not updated. How can I do that?