I need to upgrade my operating system but the display won't work. How can I upgrade ?

My 2014 MacBook Pro when powered up will not actually turn on. The display is VERY faint and in fact just a different shade of shut down. It does however make the usual start up sounds, without any display, when I start it up. When in this mode the caps lock will not illuminate when depressed. Apple support has suggested that a software upgrade may be the problem. How can I upgrade without a screen ?