I think that you use wrong approach. You should update the data only from cellForRowAtIndexPath method. Change the class variable self.milk1 in the stepperPressed method and after call method reloadData on the tableview.
- (IBAction)stepperPressed:(UIStepper *)sender
{
NSString* strval = [NSString stringWithFormat:@"%.2f",sender.value];
if (sender.tag == 0)
{
[self.milk1 setValue:strval forKey:@"Quantity"];
}
else
{
[self.milk2 setValue:strval forKey:@"Quantity"];
}
[self.tableView reloadData];
}