Sometimes you might find that your logon to your TFS service is
automatic because the credentials have been remembered by Windows. In
this case, it's not possible to switch to another user (although you
probably don't want to do that).
If you wish to log in as a different user, you need to ask Windows to forget your credentials to TFS. To do this, all you need is go to Control panel\User Accounts\Manage Your Network Passwords (or Control Panel\User Accounts and Family Safety\Credential Manager), select the Team foundation Server and choose remove. After that, you will be prompted for a new set of credentials when you go into Team Explorer the next time.