Reinstall on Raspbian (Debian Stretch)

I have Debian Stretch installed on USB booting Raspberry Pi and working well. I installed via the steps in and was able to connect with the desktop client from two of my Macs. I then upgraded to version 15 not knowing that I may have needed to proceed through v14 first.

At the end of the upgrade, I was met with the window that I needed to upgrade to 15.0.8 but the upgrade window gave an error and could not complete. Now, when I go to raspberrypi.local/nextcloud or (my internal static IP), I get HTTP ERROR 500. is unable to handle this request. raspberrypi.local shows the index of / including nextcloud/ which links to a black browser screen.

I come from a Mac world with some intermediate experience in Terminal. My Linux experience comes from the Raspberry Pi where I am a novice.

I can initialize my USB and begin with a new Debian install if that is better than undoing the mess I may have created.