deleteCharactersInRange Help

I'm trying to get this code to work. But am at a loss as to why it does not.
The code does add the neg sign "-" if it's not there. But does not remove it if it's there. Please help as I'm trying to get my head around this as a hobby.

Code:

- (IBAction)NegPlus:(UIButton *)sender {
    NSRange hasNegSing = [self.display.text rangeOfString:@"-"];
    NSString *myDisplay = self.display.text;

    if(hasNegSing.location == NSNotFound){
        
        myDisplay = [@"-" stringByAppendingString: myDisplay];
        self.display.text = myDisplay;
    }
    else {
        NSLog (@"the mydisplay before = %@", myDisplay);
        
      
      deleteCharactersInRange:[myDisplay rangeOfString:@"-"];
       
        NSLog (@"the mydisplay after = %@", myDisplay);


    }

no warning it runs but just won't delete the "-"

 

no complier error. it just won't remove the minus sign !