Rebuilding WP after a failed update

I was running WordPress 3.7 and became concerned that my WordPress version was so out of date that as some point, it would fail. The automatic update did not work, so I went through the manual update procedure for version 5.4.1. Everything went fine, but when I tested the updated version, I got a page saying the PHP version on the server I was using (which I do not control) was too early to be compatible with WP 5.4.1.

I had a backup of everything in the WordPress folder and I copied it back. When I tried the site, I got a page saying I did not have permission to read /wordpress/. I checked several sites and looked at .htaccess files and set permissions on all the files (644) and directories (755). Still no permission.

I’ve copied everything out of the way and installed WP 5.1.5, which works fine. I have the backup of all the files and the sql database whose copy I automatically make weekly. Is there a procedure for adding the pervious content to my 5.1.5 version? Alternatively, how do I solve the permissions problem?

An added wrinkle: the server I use is a central service with two factor authentication, so I cannot use FTPZilla or cPanel. I can do things from the linux shell.

Peter

The page I need help with: [log in to see the link]