Complete written instructions are in fact in the video description, too. Here they are, for first timers 😉

1. While the Steam Deck is powered off, hold the 3dots (QAM) and turn on the Steam Deck.
2. The recovery menu will appear. On your keyboard highlight the 3rd option - CURRENT (OS Boot Menu) then press enter.
3. The GRUB menu will appear. Highlight the 1st option - SteamOS then on your keyboard press “e” to edit the boot options.
4. Press down cursor on the keyboard until steamenv_boot is highlighted. Press “end” to go to the end of the line.
5. Enter the command - systemd.debug_shell
6. Press CTRL-X to boot!
7. Once SteamOS loads, press CTL-ALT-F9 on the keyboard to access the root debug shell.
8. Enter the command - passwd deck
9. Enter new password and retype the new password.
10. Once done, press CTL-ALT-F1 on the keyboard to go back to game mode.