The BIOS and other downloads can be found through here for your t620 model after selecting the OS: http://h20564.www2.hp.com/hpsc/swd/public/readIndex?sp4ts.oid=5404709&lang=en&cc=us
You have Win10 IoT as OS? The BIOS is not listed there yet.
You can find the latest BIOS from August 2015 here: