Down at the very bottom of the page is a "Contact Us" button. It is the furthest left on a line of items to click on. Use it to contact the administrative staff. It takes an administrator to do a name change, regular moderators cannot do it for you. You will be in a queue, and someone will get to you as soon as they can.
If you use the red outlined white triangle with the "!" in it to report your post, it will go to the moderators, who will try to get an administrator involved. So you can really try it either way, but the Contact US button will probably be quicker. In your request, please state what you want your new screen
name to be, or you'll get something random.