Tumblr updates its blogging platform periodically and sometimes changes its user interface; the developers may change the position of various elements, such as the Log Out and Log In buttons. However, Tumblr always provides the means to log in and out of the system.
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Logging In on Tumblr
Open Tumblr in your favorite Web browser and click the Log In button. If you don't have a Tumblr account, fill the Sign Up form and click Sign Up to create a new account.
Type your email and password into the Email and Password fields and click the Log In button to log in to your Tumblr account.
Logging Out of Tumblr
Click the Account icon — this is in the top right corner and looks like a person's silhouette — and then click the Log Out link in the Account menu.
Click OK to confirm the action and log out of Tumblr. If you change your mind and want to remain logged in, click the Cancel button.