Belated happy birthday means you are late wishing someone a happy birthday. Happy belated birthday means that someone's birthday came late, which is usually not the case. Belated refers to something delayed or late, such as a belated apology.

How do you use belated correctly?

It applies to something that is late or perceived as late. For example, if you received Christmas gifts after 25th of December, they would be belated gifts.

Is it the belated or belated?

birthday and you are sending your best wishes late, there are many options for how to say that you're sorry you're late and to wish them a happy birthday nonetheless. However, if you're choosing to use the word “belated,” there is only one correct way of doing it: belated happy birthday is the right formulation.