The best answer that I can give, is: don’t update any file!
The reason for this is that your changes will be overwritten every time WordPress updates the plugin. And you would have to do your changes again.
There are basically 3 options. Which one suits you best, depends on your skills and how you handle other changes or debugging. All options have there pros and cons.
1. Custom CSS in theme
In the Appearance menu of WP there is a Customize option and that has an Additional CSS field. Works great for small changes.
2. Plugin
There are plugins that allow you to add custom CSS (or even SASS/SCSS) and javascript. If you have more (complex) changes, this may be better than the first option. But I like option 3 better, because you don’t have the dependency on another plugin there.
3. Child theme
A child theme is very easy to set up and allows you to add CSS (and even other stuff) to the parent theme.
For all options the first step is to locate the CSS rule you want to change (use your browser’s dev tools for this) and then write a more specific variant for it. Add that rule to the option of your choice and test it.
