Add or Update Inner List in C#

Hello. I have Cars collection. In the collection, I am storing Cars that consist of id and IReadOnlyCollection (In MongoDbCompass it is a array.). In model class there are a two string properties (string Brand and string Country). In API i am receiveing a new IReadOnlyCollection. If received IReadOnlyCollection elements existBrand within Cars collections array i want to update Country field belongs the Brand. If Brand is not exist in Cars collection array, i want to add this Model to the Cars collection array? I can update array elements. However, in upsert (add) i am getting an error. Please help.