Disable New Action Center Sidebar and Restore Previous UI
1. In Registry Editor, go to following key:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\ImmersiveShell
2. In right-side pane, change value of UseActionCenterExperience DWORD to 0
3. Restart, log off or restart Explorer as given here to take effect and it'll disable the new Action Center UI.
If you want to restore new UI, change value of UseActionCenterExperience DWORD to 1 again.