how to remove freebsd when it is running

my freebsd 13 is installed at flash disk of dell thin client

it doesn't work well, i want to remove it

"dd if=/dev/zero of=/dev/ada0 count=1000 ..." doesn't work, freebsd 
doesn't allow

according to dell manual, i shall be able to enter bios by pressing F2 
before bsd booting so that i can set boot sequence  to boot from usb 
disk, but i can't enter bios, pressing F2 doesn't work

it's not easy for me to open case and dismount disk