Go to "My Account" on these forums... Or on Impulse.
I don't recommend changing it though, as I had an issue where when I changed mine I could not log on to Impulse to DL the updates, I had to rechange it back to DL it.
Also the ingame name never showed up actually in game, only on the forums...
Also Impulse will, at least for me, try to auto log you in under the original name, which in turn was the reason I could not DL patches, even if I manually logged it out and retyped the new name and logged in, the updates never showed up til I changed the name back to the original.
