What the fake xbox update screen is
Xbox updates use a dark screen with the green accent and a friendly line of text that is entirely at odds with how long the wait actually is. That gap between tone and reality is the joke.
Like the PlayStation version, this works best on a television. On a laptop it reads as obviously misplaced.
Xbox update prank screen online
Open it in a television browser, or plug a laptop into the TV, and press F for fullscreen before anyone walks in.
The console is not touched in any way. This is a page on a display.
How to run it, and how to stop it
Press F for fullscreen, then F11 to hide the browser’s own toolbars. That second step is the one that matters: an address bar sitting above the screen gives the whole thing away instantly.
To end it: Escape leaves fullscreen, F11 brings the toolbars back, and closing the tab removes it entirely. Nothing is installed, no file is written and no setting is changed, so there is nothing to undo afterwards.
Worth saying plainly — use this on people who will find it funny, and on machines you are allowed to touch. Show them the way out once the joke has landed.