How do I update a record without changing certain fields?

How do I update a record without changing some of the existing values in fields? Would I need to use a query, or CKModifyRecordsOperation?


The reason is that the fields I want to leave unchanged are of type Asset, and are large and take longer to download and upload. I want to be able to modify the other fields in the record without having to download or upload the data in those fields with type Asset.