We do not know the exact reason it may have declined, but it’s usually that you entered a digit wrong or that you put in the wrong expiration date.
The best way to handle this is to just put another order in. Check and re-enter your CCard info in My Account before you complete your order.
Changing your CCard info on a pending or declined order will NOT fix it. That order is bound to the CCard info you used when you placed it. You need to update the info then push a new order through to use that updated info.