How to change authui.dll for custom login screen

1. Go to C:\Windows\System32 folder and make a backup of your old authui.dll with a name like authui_old.dll

2. Take ownership of the file. Tutorial for this step.

3. Open any external file manager (I used Total Commander) in Administrator Mode.

4. Locate C:\Windows\System32 in one tab and your new authui.dll in the other tab.

5. Open Task Manager, switch to Processes tab and close explorer.exe.
NOTE: your taskbar and desktop icons will disappear, but don't worry, it is normal.

6. In Total Commander copy the new DLL to System32, hit overwrite.

7. When finished, switch back to Applications tab in Task Manager, hit New Task... and enter explorer.exe, then OK. Everything will come back.
NOTE 1: If you closed Task Manager after Step 5, press Ctrl + Shift + Esc to open it back.
NOTE 2: If you have customized start orb picture, you will have to apply it again.