You have a new SSD drive and want to upgrade steam deck (SD), you must do:
1) install new SSD drive to SD and get out old SSD or emmc drive (64GB)
Be careful with SD plastic case, its very fragile. You better start disassembling the case from the top side.
2) Download Stem OS SD special image
3) Burn image to USB drive with rufus - change MBR boot type
https://youtu.be/6vFmOf4V6O4
4) Power on SD holding (-) volume down key to boot into bios mode and choose boot manager - then choose USB drive
5) Boot from usb and choose "rewrite image" script icon
6) Unplug USB drive and reboot
