If you want to keep your changes you should move all the changed files into child theme and move changed styles only into child theme style.css (Don’t copy just all the parent theme style.css into child theme style.css, only changes styles!)
Do the backup of all your files, DB and theme options (Theme Options > Import/Export > Options > download export file). Only after that, you may try to update theme.
Actually, there are many services to compare files, for example https://text-compare.com/ You can add source code of style.css file and your style.css file and check what changes you did.
But I’m not sure about folders with files if you changed not just styles but other files. You can try this one, for example https://www.devart.com/codecompare/ or look for similar.
Also, I have sent you original archive with 3.0 version. You may compare that archive with your theme to find out what changes you made.