how to delete unused CSS ?

Hello there.

I am doing a site performance improvement and I came to CSS file and I found jetpack has many unused CSS as google recommended to remove unused CSS and pointing at jetpack CSS.

I located the unused CSS from jetpack and deleted them manually, the question here is how to keep the edited CSS file away from updating. When the plugin has an updated I am sure that it will generate-change my current CSS file that I just did. is there a way to keep the file while updating the plugin, or how to keep it away from updating? do you have a better safe way to remove unused CSS of the plugin?

Thank you