If it's still not done then I'd see if you can quit the installer (force quit is command-option-escape).
Next I'd make sure that there is a backup.
Then I'd boot into the recovery partition and repair your hard drive (repair permissions while you're at it.)
Then try the Update again.
Or you might want to wait a few days for Mavericks to come out.
You might fare better with the Combo Updater. OS X Mountain Lion Update v10.8.5 (Combo)