How to implement & save custom changes made on cPanel to a plugin

Hi guys

This is the first time making a post on this forum, so my apologies for any errors.

I am a beginner level Word Press user for my company and I received an update for a buggy plugin. I stupidly pressed ok to the update which had been altered within the cPanel beforehand, this is the message I received below.

Core Calendar Themes are out of date. We have found updates for some of your core Calendar Theme files and you should update them now to ensure proper functioning of your calendar.

Warning: If you have previously modified any core Calendar Theme files, your changes will be lost during update. Please make a backup of all modifications to core themes before proceeding.

Once you are ready, please update your core Calendar Themes.

How do I go about finding/saving such changes so I can implement them for the updated plugin.

Thank you