I changed mine from NeonSiren to Metaphor a few months ago. Email contact@myvmk.com and include your current in-game name, then the one you want to change it to. For example, I just wrote something along the lines of: "My in-game username is NeonSiren. I want to change it to Metaphor. Thank you."
Make sure to use the email you used to create your account (otherwise people would be changing other accounts' names and that is rude).