according to IBM documentation you're supposed to "contact an authorized service provider" to reset the admin password
but, I asked the seller and they told me that there's two switches on the FSP (Flexible Service Processor) card and that you can reset the password by changing the positions of both switches, booting the server (or rather the FSP), then changing the switches back and rebooting it again. I did that and it worked! the switches are tiny and covered by yellow translucent tape so I didn't notice them at first