How to use TFSAdminUtil in Order to Change TFS Service Credentials
1- Open the Command Line:
a. Start/Run then type cmd
b. Navigate to the following path:
%programfiles%\Microsoft Visual Studio 2005 Team Foundation Server\Tools
c. At the command line, type TFSAdminUtil ChangeAccount oldAccount newAccount newPassword, and then press ENTER. You must enter the user name for the old Team Foundation Server service account (oldAccount), in addition to the user name and password for the new account (newAccount and newPassword).
For more details about TFSAdmin please read my article :
Changing the Team Foundation Server Services Accounts Users and Users’ Credentials