To assign a record an Owner you have to send an Assign Request using the Execute method.
service.Execute(new AssignRequest()
{
Assignee = new EntityReference("systemuser", userId),
Target = new EntityReference(updateEntity, recordId)
}
);
The Assign Request has two properties:
- Assignee: This can be either a Team or a User.
- Target: this is the record you want to assign.
Hope this helps.