Best way to upgrade Enfold since 3.5.4

I have been given a website with Enfold in version 3.5.4. The first thing I did was to create and activate a child theme, importing the settings of the parent theme. I tried to upload directly the new Enfold version 5.6.1 folder via FTP (renaming first the folder of the old theme with enfold-OLD) but it didn’t work.

So I entered the user and the themeforest API Key in the Enfold section but it doesn’t detect updates. And if I click on the “Check Manually” link it returns these errors:

Warning: Illegal string offset ‘http_code’ in /homepages/11/d223943919/htdocs/wp-content/themes/enfold/framework/php/auto-updates/class-envato-protected-api.php on line 307

Warning: Invalid argument supplied for foreach() in /homepages/11/d223943919/htdocs/wp-content/themes/enfold/framework/php/auto-updates/class-pixelentity-theme-update.php on line 49

Warning: Illegal string offset ‘http_code’ in /homepages/11/d223943919/htdocs/wp-content/themes/enfold/framework/php/auto-updates/class-envato-protected-api.php on line 307

Warning: Invalid argument supplied for foreach() in /homepages/11/d223943919/htdocs/wp-content/themes/enfold/framework/php/auto-updates/class-pixelentity-theme-update.php on line 49

Warning: Cannot modify header information – headers already sent by (output started at /homepages/11/d223943919/htdocs/wp-content/themes/enfold/framework/php/auto-updates/class-envato-protected-api.php:307) in /homepages/11/d223943919/htdocs/wp-admin/admin-header.php on line 9

The website is working on PHP 7.3 and I can only make the jump to PHP 8.1. It is also working with WordPress 5.7.8, but I think I should not update WordPress until I have updated Enfold.

I would appreciate any recommendations you can give me. Thanks.