How to reload the previous view controller on back button?

You have to call your API method in viewWillAppear(). So, whenever you come back from any viewController, your updated server data will reload itself.

I assume you are sending a web call from viewDidLoad(). So in order to update your viewController A Add your web call method in viewWillAppear()

The state of your "data" on view controller A has not changed and you need to refresh it. If you're fetching data for view controller A within -viewDidLoad, try moving it to a different life cycle method like -viewWillAppear.

Remember, -viewDidLoad gets called only when the view controller loads which happens once, however -viewWillAppear gets called before it appears which will happen often if you're hopping back and forth between views.