Quote Originally Posted by lat9 View Post
Just follow the zc155e upgrade instructions documented here. You'll need to add a couple of notifications to the PayPal Express Checkout payment method, too (see the section in that documentation just above the link I posted).
You are pointing to the instructions for Zen Cart 1.5.4, 1.5.5, 1.5.5a-f, (under the Perform Required Edits) but there is also, a bit higher on the page, instructions for Zen Cart 1.5.5b-f (under the Installation), and even more at the beginning, there is this mention:

If you are upgrading an existing OPC installation from a version prior to v2.0.0, you'll need to uninstall that previous version and perform a fresh installation of OPC v2.1.0.

And the uninstallation instructions indicate to delete the files and then run the SQL patches.

So back to my original question: How do I know which files to delete before running the SQL patches?