You will need a USB type C to type A adapter and a USB hub. Flash the SteamOS recovery image (or any other distro) to a flash drive, noting that it will erase all the data on it. Plug a keyboard and that flash drive into the Steam Deck, then hold volume down while turning it on to get into the boot menu.

For chrooting instructions, read this: https://linuxcommandlibrary.com/man/steamos-chroot

Your drive is going to be /dev/nvme0n1

You may need to switch into your user profile after chrooting if the first systemctl command doesn't work. I think the username in SteanOS is "deck", so the command is "su deck"