When new versions of CSS or JS resource files are uploaded to a live page, properly set up cache settings often take more than 2 weeks until the browser reloads the files.
I recently saw the strategy to add a query string to a CSS resource in the HTML tag like this:
Assuming you have your CSS/JS somewhere in your site packages Resources/Public folder,
the most straightforward way would be, to create an own ResourceManagement Target, most likely by just extending the FileSystemSymlinkTarget.
Then overwrite the getPublicStaticResourceUri() method, to return an URI that contains a version parameter.
Then configure that Target for your Neos.Flow.resource.targets.localWebDirectoryStaticResourcesTarget in your Settings.yaml and use {f:uri.resource()} viewhelper in your templates or equivalent Fusion, if you don’t do that already.
If you have your assets stored somewhere else, you can still do it that way, but you need to define the resource collection, storage and target yourself and configure everything to make it work.