Hi there!
Thanks for your query.
Actually, it is not recommended to edit/modify your template.css file. You can use the custom CSS field within the Custom Code tab of your Template Options.
Or, you can create a custom.css file and keep all your custom CSS codes within the file. Then it will not be overrided with the updates.
Here is the documentation link:
https://www.joomshaper.com/documentation/helix-framework/helix-ultimate-2/custom-code-css-js-meta
If you are interested with the view override of the template then here you go:
https://www.joomshaper.com/documentation/helix-framework/helix-ultimate-2/template-views-override
Hope this helps!
Best Reagrds