Best way to upgrade your old Mac Pro to macOS Sequoia 15.5 for 2009-2013 models
Once upgrade you will be able to continue to apply the latest updates. For Mac Pro below you will also need a USB 2.0 hub. Mac Pro Early 2009 (MacPro4,1). Mac Pro Mid-2012 (MacPro5,1). Mac Pro Late-2013 (MacPro6,1).