I don't think there's a method to log out. After a certain amount of time you'll have to log in again, so you could enter a different account, but it's easier to just clear your browser cookies or use a private/incognito browser window.
Edit: @HDMIGuy 's response is much better, although it's for this "community". For my.roku.com you can sign out using the menu in the upper right corner. It's under "Welcome back".
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