You can either logout from the QMC manually or be automatically logged out. Automatic logout occurs when you have been inactive in your QMC session for longer than a predefined time limit. This time limit is set per virtual proxy in the Virtual proxy edit page.
Do the following:
Click usernameS in the top right of the page.
Logout is displayed in the drop-down list.
The QMC welcome page is shown including a Login button.