compatibility between Astra theme and visual composer plugin

My web site was originally built with astra theme. Suddenly, it was impossible to modify the text on the pages or articles because all was encoded by Visual composer plugin and unreadable! In this condition, how update the information? So I tried to update the visual composer plugin. I paid for that, but the plugin doesn’t work.
The error message say:
> "Fatal error: Cannot redeclare wpb_getImageBySize() (previously declared in /home/clubmoa/www/wp-content/themes/Astra/wpbakery/js_composer/composer/lib/helpers.php:15) in /home/clubmoa/www/wp-content/plugins/js_composer/include/helpers/helpers.php on line 114"

The visual composer plugin editor says that:

“1) The issue is because there is a Visual Composer related code present in your theme added by the theme author. You will have to get rid of it. I would advise you to do so with the help of theme author, otherwise you might loose the extra elements added by him. Once this is done, you can install the standalone plugin and it will work fine. Kindly check.
2) The issue is because you are using an very old version of visual composer which is 3.7 and should update it to the latest version which is 5.0.1. You need to deactivate and delete the older version of the plugin from theme and then install the standalone version which is purchased by you. However, I would also suggest you to re-confirm the same with the theme author before proceeding, because if he has done any changes to the core plugin files, then they will be lost during this process, which in-turn might affect the content/layout at your end.”

So, my question is where/how to do this. Someone could help me?

I too need help with this and have contacted the author twice and do not have a reply from a week or more. How do you get someone else to help with this?