how to upgrade the dell optiplex gx620 bios upgrade
you can download the bios at dell homepage like here http://support.dell.com/support/downloads/download.aspx?c=us&cs=19&l=en&s=dhs&releaseid=R142230&SystemID=PLX_GX620&servicetag=&os=WW1&osl=en&deviceid=162&devlib=0&typecnt=0&vercnt=8&catid=-1&impid=-1&formatcnt=0&libid=1&typeid=-1&dateid=-1&formatid=-1&source=-1&fileid=189785
that's a download for home use pc and before you download i recommend that you have something like a ups (Uninterruptible power supply) to if the electricity gone cause if that happen while you are upgrading your motherboard you cannot use your old motherboard anymore, that mean buy a new one or don't worry if you don't have a ups.
next thing after you have download(via your browser). run the file and click continue and do what it says and click on ok and it may be ask for your administrator password wait after you see your pc shut down and reboot do not touch nothing on your PC while this happen wait again after a message say upgrade successful and your PC reboot automatically and you are done!