You would have to cancel your current order and have CS place the order for the color you want. Contacf them if you haven't already. Unfortunately depending on what you switch to you may still end up not getting it on the 16th. You can also try your luck on launch day and see if you can get what you want...and if they have it cancel your preorder if it isn't being prepared for shipping. I've done that before with no issues and did it for the iPhone 6s for my mom on launch day.