upgrade crashed and now I'm stuck

I ran the upgrade in Software Updater to move from 16.04 LTS to 18.04.2 LTS. It reset sources and when I clicked to continue, it gave an error that it was unable to lock a file. Then it aborted. It did not revert. Now when I try to run Software updater, it just crashes. No error message.

I used Sources in settings to remove checked boxes for "disabled for Bionic upgrade" and I set back boxes for Xenial. No changes. When software Updater runs it just crashes.

Next I downloaded the new software for 18.04.2 LTS as an ISO, burned a DVD and booted. There I have a option to Install Ubuntu but when I run that, I have no option to upgrade. I have the options to erase the old version, along with all of my files and programs, and install a fresh copy of 18.04 or to install along side and choose which version to run each time I boot. This needs more disk space than I have and isn't what I want either.

How do I proceed?

Thanks and I did try to find the answer to this in the online documents but can't find the answer.