please read at first the informations on the main page here:http://www.jdownloads.com/index.php?option=com_content&view=article&id=231:urgent-security-update-for-19-series&catid=51:news
Then should you install directly the security update vor your installed jD version (126.96.36.199).
Please note that the 1.9.x series is not the newest jD series and will be replaced with the newer jD series 2.5/3.2.
But it exist in the moment not all jD plugins and modules also for the new series. So when users need a special jD plugin or module which is not included in the new installation packages, he should wait a little bit more with the update.
All other users should fast update to the newer series. Update instructions can you found in the documentation.