How do you get the creases out of a 1917 US 2 dollar bill?

It's best not to try. A circulated 1917 US $2 bill can be worth anywhere from $50 to $100. Anything you could do at home (ironing it, pressing it between books, etc.) might actually do more harm than good to its value as a collectible. You'd need to consult with a currency dealer who can examine the bill in person and determine whether any change would be safe.