Fatal error after upgrading from 3.4.7 to 4.7.6.3

Hi guys,

How can I upgrade from 3.4.7 to 4.7.6.3?

I’m working on a website which has Enfold 3.4.7 installed.
After the latest WordPress update, some parts of the theme aren’t loading any more.

Also, when checking the theme’s status in the backend, WordPress doesn’t recognize that the theme is outdated an in need of an update.
That was already a bad sign.

So, I purchased a new licence for Enfold. However, after uploading via FTP, both the site’s frontend and backend said it had a critical error and the site was inaccessible.

WP DEBUG says the following:
Fatal error: Cannot redeclare av_countdown_events_fallback() (previously declared in /home/w7ur5khswoym/public_html/wp-content/themes/enfold/config-templatebuilder/avia-shortcodes/events_countdown/events_countdown.php:16) in /home/w7ur5khswoym/public_html/wp-content/themes/enfold/config-templatebuilder/avia-shortcodes/events_countdown.php on line 8

For now I just reuploaded the theme…… But I really need to figure out this issue.
Any ideas?