This article pertains to how to log out of PeopleUpdate. PeopleUpdate uses Windows authentication and, depending on your browser settings, the credentials you used for the presentation may be stored by your browser. If you use Internet Explorer, you can go into Tools > Internet Settings and select the Security tab. Choose the Local Intranet (or possibly Trusted Sites) security zone and uncheck Automatically detect intranet network. This should prompt you for new credentials next time you go to PeopleUpdate and allow you to change the identity.
Once you have your identity set, you can change the Intranet security zone setting back to the default value.