Change the CSS of a plugin

Hello there, I recently installed the plugin “Instruct“. Thanks to the author, great plugin.

However, I want to change the CSS of this plugin to make images of the plugin largest and change the color of the cells.

But when I change this plugin CSS located in the folder “/public_html/wp-content/plugins/instruct” the changes made ??do not appear.

I try to add my custom CSS version of this plugin in the CUSTOM CSS of my theme but the changes made ??do not work, as if I would have let the original CSS plugin version …

How do I edit the CSS of a plugin located in this folder /public_html/wp-content/plugins/instruct

Thank alot!!