Upgrade from Yosemite to a higher osx

I sold my iMac from 2012.

I installed osx from scratch,it was mountainlion.

After getting an update, the mac installed Yosemite.

Now the iMac is sold to a collage.

He can't get on most websites using safari.

I read that it has to to with on old version of safari and he had to update the osx.

BUT he doesn't get any question mark to update his osx!

He has a few updates to install, but when he tries it he get error 102.

The only update that was successful was safari.

How can he upgrade to a higher osx?