Similar to adding a new custom sub-entity row you can get a CustomSubEntity from a CustomEntityList and change the column values for the row.
Just like adding rows you can change the field values either by using a CustomEntityFieldDescriptor’s SetValue method or by using the indexer on the Fields property of the CustomSubEntity object.
There are also methods on CustomSubEntity for setting the access level of the row and changing the parent records.
None of the changes to the row are saved to the database until you call the Update method.