if I update the headers of the last x days and I can download them
It'll update them if the headers are downloaded. By default Newsbin just downloads the newest headers in the group. If you want to make it go backwards and download old headers again (which is what this sounds like) it can do it but, you have to tell it to actually do it.
You can do this by setting a "Download Age" for the days of headers you want to re-download, then right click the group "Post Storage/Use Download Age" then download headers from the group. This will make it re-download X days worth of headers and update the old records.