How do I change a user name in Windows 7?

Make a new account and do a settings and data transfer using Windows Easy Transfer from account to account! Delete the old account. All in all this will not take long depending on the size of your profile.

"Try the procedure described here : ""How to rename or move a User Profile folder ?"".

Renaming the user profile folder

To rename (or move) the User Profile folder, you may use the following method. This method has been adopted from KB314843, but this page explains in detail how to rename an User account home directory.

Important note :
Though you can move or rename the user profile folder, there may be some side effects after using this method. This is because of the reason that there may be some absolute path references (to the old user profile folder) in the registry added by third-party software. Therefore, there may be a loss of functionality in the respective applications.

Before modifying the registry, you need to take a full backup, in case you want to Undo the changes or to recover from disasters caused by incorrect registry modifications if any. You may use ERUNT for a full registry backup. Use Registry Editor at your own risk.

Rename the User Profile folder using Windows Explorer:

Logon to an admin account that is not the account being renamed.
Open the Documents and Settings folder, by typing this in Start, Run dialog:
%systemdrive%\Documents and Settings
The list of folders will be displayed. Select the corresponding folder of the user account that you want to rename.

Example:

%SystemDrive%\Documents and Settings\OldUsername
becomes:
%SystemDrive%\Documents and Settings\NewUsername

Next step is to notify the system that the user profile path has changed.

Changing the ProfileImagePath value in the registry

The ProfileList registry key contains some sub-keys, which are nothing but the list of User Account Security Identifiers (SID). Each of the SID represents an Account. The key is located here:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList

Identify the SID for your User account, and change the Profile path

To know the SID for your user account, you may use the script sidlist.vbs
Download the script and run it.
(The User Account names and SIDs will be listed in a log file, and opened automatically.)
Note down the SID for your account.
Then, in the Registry Editor, select the correct SID that belongs to your user account.
In the right-pane, double-click the ProfileImagePath value and change the profile path. (ProfileImagePath stores the full path of the User account home folder. )

Example:

%SystemDrive%\Documents and Settings\OldUsername
becomes the following:
%SystemDrive%\Documents and Settings\NewUsername

Close Registry Editor, and restart Windows. See if you're able to logon to that user profile successfully. Additionally, to verify if the path has been changed successfully, type SET in the Command Prompt. In case you find any abnormal behavior when running an application, you may undo the above procedure."

"According to my own experiment, there're mainly 4 steps you have to do to change your account's login name safely as follows:

Make full disk backup or create a disk restore point for disk C. This may be tedious but definitely necessary.
Change your login name in various ways.

Running netplwiz as user40662 has pointed out.
Using Computer Management tool by running compmgmt.msc. Just modify your user settings under Local Users and Groups.
wmic useraccount where name='currentname' rename newname as Sriniv has pointed out although I didn't test it.

Modify your user profile name and change the registry related. harrymc's answer is worth reading although not perfectly right due to the old Documents and Settings. Or you may refer to How To Change A Windows Account Name And User Profile Folder Name - gHacks Tech News

To rename your user profile, you have to create another temporary administrative account and login to it.
Rename your user profile C:\Users\username to C:\Users\newusername
Locate your own ProfileImagePath under the registry directory HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList and change the value to your new profile directory name. Be careful not to change the wrong one.
Logout and relogin with your new account name and delete the temporary account.

Post processing. It's predictable especially for some path settings.

You may have to selectively replace C:\Users\username with C:\Users\newusername in system registry if needed. Do a fully backup before massive modification!
For cygwin users like me, you need to run mkpasswd -l > /etc/passwd;mkgroup -l > /etc/group to refresh users and groups, then change the user directory name as expected."

"Do the following:

Run the command control userpasswords2 to open the User Accounts dialog
Select the user account to rename and click Properties
Enter the desired username in the User name field and click OK
Log out and back in if changing the account currently in use

Note: This does NOT rename the account's user profile folder."