Question: I have a navi-controller(A) with buttons.

As a button, a modalview(B) appears.

In modalview(B) I can access a button to move another navigation Controller(C).

How can I code?

Answer: 
You can dismiss a UIViewController that was displayed modally before displaying another UIViewController. Or you can display another UIViewController on top of the first view controller. `self.navigationController` will probably be useful.
try this to open a new modal for both the modal set this action for the buttons you want.
 -(IBAction)btnAddNewPressed:(id)sender
{
 ViewController \*s= [[ViewController alloc]initWithNibName:@"ViewController" bundle:nil];
 [self presentModalViewController:student animated:YES];

}