If you renamed your account, and want to rename it back to the original name, you can do that.
IMPORTANT: This may not work if you are outside of the US and have a "@live.com" or "@Hotmail.com" address!
First, remove the alias that was created when you renamed your account (the alias will be for the original name).
Go to http://account.live.com
, select "Notifications" then "manage your email preferences" and you'll find links to remove aliases.
Second, go back to the Rename page, Sign In
and rename back to the original name.