UITableView edit propagation to the model in MVVM - ios

Question

I'm creating contact application learning how to use ReactiveCocoa with MVVM. I want to implement edit mode for the contact details table, so that when user taps Done button, I save these changes to the model.
Currently I have this structure:
List Of Contacts table has VC and VM, each of its cells has custom view and VM.
When user taps on cell, I open details table.
Contact Details Table has VC and VM, and again each cell has it's own custom view and VM.
When user taps Done button in Contact Details I can observe that in it's VM and construct new Contact object, but how can I notify List Of Contacts about this change?
I'm very confused with such architecture at the moment. So what is a common way to solve this user case in MVVM?
Update:
I have updated the code and solved the problem, however I don't like how it is solved and the question is still up.
When user taps a cell on a contact list, new Details ViewModel is created. I pass a reference to the ContactsList ViewModel and index of selected contact in order to handle contact updates. (Later on if contact is changed, I change contacts property of contactsList view model)
ContactsList ViewController:
-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender{
if([segue.identifier isEqualToString:#"pushContactDetails"]) {
STContactDetailsViewController *vc = (STContactDetailsViewController *)segue.destinationViewController;
vc.viewModel = [[STContactDetailsViewModel alloc] initWithContactIndex:self.viewModel.selectedContactIndex services:self.viewModel.services rootVM:self.viewModel];
}
}
Details ViewModel:
-(instancetype)initWithContactIndex:(NSNumber *)contactIndex services:(id<STViewModelServices>)services rootVM:(STContactsListViewModel *)rootVM {
//some initialisation
#weakify(self);
self.saveChanges = [[RACCommand alloc] initWithEnabled:[self executeChangeCheck] signalBlock:^RACSignal *(id input) {
#strongify(self);
return [self executeSaveChanges];
}];
}
-(RACSignal *)executeSaveChanges {
#weakify(self);
return [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
#strongify(self);
STContact *newContact = self.contact;
//Update newContact with changed values
NSMutableArray *newContacts = [NSMutableArray arrayWithArray:self.rootVM.contacts];
[newContacts replaceObjectAtIndex:_contactIndex withObject:newContact ];
self.rootVM.contacts = newContacts;
return [RACDisposable new];
}];
}
When contacts property of ContactsList ViewModel is changed, its table reloads and I can see these changes when I get back from details view.
Moreover, In Details ViewController I handle view updates of its table if contact is changed and apply them when exit tableview edit mode.
Even though, everything works at the moment, I feel bad about passing reference of the ContactsList ViewModel to the Details ViewModel. I'd be grateful, if someone could show me the right way to handle this kind of situation.