You can use the SAMD21 update payload in the Switchboot thread. Send that via TegraRCM or whatever, then double-press reset on chip.
Oh I'm able to get into update mode fine, but I just wasn't sure about this section:
"Check what straps you have fitted. If you are lucky enough to have the vol+ strap soldered, this is for you. Maybe.
If not, then you can still use it, but you probably will want to avoid "part 1".
I don't have vol+ strap connected, so I wasn't sure if you mean skip that part 1 altogether. It's all good now though. I did update both and everything is working fine. Thanks so much!