Pass data between same view controller class

I've used a push segue to get to the next viewcontroller form a viewcontoller.

Push segue pushes a brand-new instance of the view controller, meaning that the view controller currently on the top would have only the values set in itself, while values set in all of its predecessors would be nil.

You should not be relying on storing values in fields of your view controller. Instead, you should set them on the shared instance of your model class, as suggested by the Model-View-Controller design pattern:

class Model {
    static let sharedInstance = Model()
    var fName : String
    var lName : String
    ...
}

When you detect that the next view controller is about to open, store the state accumulated by the current one in the shared model object:

Model.sharedInstance.fName = self.fName
Model.sharedInstance.fName = self.lName
...

Just because data is in the "same file" doesn't mean anything to the app at runtime. You're still creating multiple separate instances of your RegistrationViewController class. Therefore, you'll need to pass data between these separate instances, the same way you would for completely different classes.

Since you're using storyboards and segues, the method -prepareForSegue:sender: is a good place to look into doing that.

For the record, I agree with @dasblinkenlight's suggestion to store this data in a model object instead of individual fields, but disagree with using a "shared instance" if you don't have to. An example of passing a model object would look like this:

User.h

@property (nonatomic, strong) NSString *fname;
@property (nonatomic, strong) NSString *lname;
@property (nonatomic, strong) NSString *email;

RegistrationViewController.h

@property (nonatomic, strong) User *user;

RegistrationViewController.m

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    RegistrationViewController *nextViewController = (RegistrationViewController *)segue.destinationViewController;
    nextViewController.user = self.user;
}

- (IBAction)doSaveAction:(id)sender {
    NSLog("email: %@", self.user.email;
}