How to update a list of objects in Realm DB Swift

Question

I'm trying to find the right way to update a list in a Realm object. For example:

class Person: Object {
    @objc dynamic var id: String = ""
    let children = List<Child>()
}

//Adding an item to the list
func add(child: Child, _ completion: (DBError?) -> Void) {
    do {
        let ctx = try Realm()

        if let person = ctx.objects(Person.self).last {

            try ctx.write({
                person.children.append(child)
                completion(nil)
            })
    } catch {
        completion(DBError.unableToAccessDatabase)
    }
 }

This seems to work for adding an element. So how do I update an individual element in the array as well as replace the whole array to ensure it persists?