You can cancel your order and place it again if you need to make changes to it. Go to www.threadless.com/orderstatus and enter your order#, email address, and billing zip. Make sure to enter your zip just as you entered it when you placed the order. If you did not enter your billing zip in the zip field during checkout, you can leave this blank.
Once you pull up your order, you will be given the option to cancel. Your card or payment option will be reimbursed and you can go ahead and place another order.
If you aren't able to pull up your order, you can also send changes to service@threadless.com. Changes can only be made if your request is answered before your order is processed. Depending on demand, we may or may not be able to make your requested change. Once your order is processed or shipped, no changes can be made.