I want to file a complaint.

I recently made a pre order for new apple iPhone and watch, I got up at 3am to make the purchase. I successfully placed the order, received a phone number for device and order number. I then get a message on my app, not an email or voicemail, that they will be cancelling my order because they needed additional information. I called an agent, and they told me no additional information was needed and that the order was still set for delivery. I didn't have confidence in that response because my order details were now blank on my phone app. I called again and a different agent told me they needed to cancel my order and reorder because the system was jammed the day of pre-orders and my order was somehow "messed up". I acquiesced and placed another order relying on the advice of the agent, but obviously lost my place in pre order priority and will not be receiving my apple watch order at all (its now sold out). My iPhone will be delayed as well. I am completely unhappy at how I've been treated and the time I wasted ordering my devices as well as after trying to fix it. I'm considering moving to verizon which has a better network. I have been a Life long Att customer now getting ready to switch. My question to this long preamble is where do I file a written complaint, so that ATT is aware of why they are losing a customer?