Hiccups are normal. You don't need to get rid of them. They are simply a part of a growing nervious system fiering and learning what to do. They are actually a good thing in that they show the puppy is developing.
Remember, they are not born fully functional but instead their nervious system along with other functions like eyes, ears, and motor control continually grow and improve until they are fuly funtional.
Another reason why 6 week old pups should be with the litter they are just not yet developed.