As far as I know, you an only see the changes in the History section of the Summary Tab, but there is no way to revert the changes made to the test cases. Refer to this post for more details on this
It's not only the issue with TFS, other Project management tools like JIRA, also not have such a feature.
But there is a workaround of doing this, if you have your old backup in excel then you can restore the same. For doing this pull the current/updated test cases from the TFS in Excel and override them with your old backup using copy-paste, keeping the ID of the test cases same. Then Publish the same back to TFS, for this you may use the utility 'Test Case Migrator Plus'.
Another way which can be used in this scenario (in case you don't have backup), is to Pull the updated test cases from the TFS in excel and then update the same in excel manually and then publish it back to TFS, because it will be time saver to update test cases in excel then in TFS directly.