Imagine you have a wooden ship and over time you replace each plank. Years go by, and every piece is replaced with new pieces. You decide to not waste the old pieces and construct another wooden ship with those old pieces. What ship is the original? The ship constructed of the "new" pieces or the "old" pieces?
You are the ship. Every molecule, cell, and atom of your body has been replaced. The atoms that compose "you" are not the same as when you were born. It feels almost paradoxical that we might say that the old pieces are still the original ship, but when we consider ourselves we ignore the old and only focus on the now.