problem to update the plugin

Hello!
I have a little problem I really unable to solve this is why I need you !

There is some time (about 1 year) I installed a WordPress site to a friend. The page creation interface was clearly combined with PageBuilder! After an update WP, the structure of interfaces became chaotic. So I wanted to update the plugin that should certainly resolve errors.

But my problem is that PageBuilder does not appear in the list of extensions! So I told myself : it must be installed somewhere in the FTP. I actually find it in the folder: wp-content/themes/mytheme/include/PB
(I can also see VisualEditor folder)

Problem: When I replace it with the new version of PageBuilder … The site doesn’t work (blank page) … I tried to add PageBuilder in extensions but obviously I have error messages (there is already installed elsewhere so he do not like: /).

How can I do to uninstall the old without errors to be able to add quietly in extensions?

Thank you thank you thank you in advance! (I’m sorry for my bad english i’m french :) )