I am using x6 and have never updated it since march. I trid and it said I was not a premium member so I dint think I could ever update. I recently found out that wasn't the case and tried to update and this is what it says.
You may be trying to update a Home and Student version using the updates for the full version, or vice-versa.
If you have X6 H&S, I don't think there are any updates for it. There may also be other special versions that require specific updates. The easy way to be sure is to check for updates from the Help menu within the program. This should look only for the applicable version updates.