You could try updating via the terminal and see if that works…
or maybe swapping in a sudo apt-get update && sudo apt-get dist-upgrade --fix-missing if the former doesn’t work. Although, with a system that hasn’t been updated in a very long time it may be needed, or at the very least desirable, to just do a clean install. At the top of the site there is a “download” link where you can find a program to make install media which you can use to wipe your box and start fresh.