Issue with upgrade

Can anyone assist?
I update a very old version of concrete5 ( to the latest legacy version (5.6.4) to export all content and
I do it manually (replacing the concrete folder and browsing index.php/tools/required/upgrade
This upgrade cause an error of the blank dashboard so I grab the js folder of the old version and replase the js folder of the new version; this fix the issue but now when I navigate to any content page I got the error:

Fatal error: Call to a member function checkLogin() on a non-object in /var/www/concrete5/themes/mytheme/show.php on line 78
Any idea?

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JohntheFish replied on at Permalink Reply
I am not sure you can do that in one step. You may need to go through one or more intermediate steps. You will need to read back through the release notes for the intermediate versions to make sure what the steps are.

5.6 updates need to be run through the /updates/ directory. They can't simply be copied over like in 5.7 or v8. Then run the 'update' in the dashboard. Once update is completed and tested, you can then copy the new core over the existing core, edit the config and remove the /updates/ to tidy up and get rid of old files.

You best bet is to revert to a backup of your site before starting this process, files and data, then go through the safe procedure.