upgraded to el capitan, can I use this to get to High Sierra?

Good News, I upgraded. I found an el capitan .dmg installer for macs that can't be upgraded, beyond El Cap. Then I found the first version of the el Capitan upgrade for use with Snow leopard. Using them both I upgraded to EL Capitan.

I need to get up to MAC OSX High Sierra to run some adobe apps. I guess I now have to upgrade the firmware? https://www.ifixit.com/Guide/How+to+Upgrade+the+Firmware+of+a+2009+Mac+Pro+41/98985#s182628

Can I get two more OSX upgrades to High sierra? Do I install Sierra then Install High Sierra?