Currently, there's no option to update test cases by importing files. What you can do is use the Edit in Bulk option to update multiple test cases that share the same data. To do so, select the test cases you want to update, then click on the More▾ button and select Edit in Bulk. This is useful if the test cases have the same value for a specific field.
If the test cases do not share any common value, then you'd have to update each of them individually.
Please give your vote and comment, so that it helps the product managers with the backlog prioritization. Also, I'll check with them if we can merge those two ideas...
In the meantime, do you think that using the API could be an alternative to updating your test cases? A script that reads the csv file and post updates using the API would do the trick. It's obviously not the most straightforward solution, but I suppose that's a good workaround while this feature is not delivered:
using the API is not an option because the ones that has to update it are not programmers. i have to create a specific task and reserve time for this and my dev-team is not to huge. But nevertheless i will take it into consideration