The download took over an hour and then installation about the same again. I am now running Apple’s latest, greatest OS X El Capitan (version 10.11). Free upgrade, of course.
I had to download and install a special “legacy” version of Java in order to keep using my old Adobe apps from Creative Suite 4. If I run into any surprise incompatibilities, I’ll come back and report them here.
El Capitan is supposed to run more efficiently, so I updated to take advantage for my aging iMac.