You need to remove TFS credentials from Windows Vault to clear and force to ask new TFS credentials in Visual Studio
Go to Control Panel (Start -> Control Panel).
Click User Accounts ( or User Accounts and Family Safety->User Accounts in Windows 7 Machine)
Click Credential Manager (or Manage your credentials)
In Credential Manager page, you can see the two type of credentials
i. Windows Credentials
ii. Generic Credentials
5.Click on two credentials modify link, click the link Remove from vault to remove stored TFS credentials.
Now, When you login into Visual Studio you will be asked to give credentials to connect TFS.
Note: Don't forgot to uncheck the option Remember my credentials to force to ask credentials for every TFS connections.