In Windows 10; Restart, Shut Down, Lock, Sleep, and Sign Out are located in different places. In earlier versions of Windows, we were using Start menu > Power > Log out, but you can see in below screen Log Out has been removed from Power. After clicking on Power, you can choose from three options : Sleep, Shut down and Restart.
The other methods to access the Sign out is to press Ctrl+Alt+Del or you can use Windows-Key+L to Lock the PC, but it is suitable for computer/laptop and not while you are using Windows 10 in Tablet Mode.
You can also access Restart, Shut Down, Sleep, and Sign Out from ‘Win X’ menu; by doing right click on Start button of Windows 10.