Follow this guide carefully:

Upgrading Procedure for an Official OS release is as follows:

1. Download the OS and install it on your PC. (Make sure you have already removed any older OSes you have installed on your PC by going to Add / Remove Programs in Control Panel)

2. Delete the vendor.xml file (located in C:\Program Files\Common Files\Research In Motion\AppLoader)

3. Backup your 3rd party applications ( optional recommended step )

4. Connect your BlackBerry to your PC and right click on a file named Loader (located in the same place where you found the vendor.xml > C:\Program Files\Common Files\Research In Motion\AppLoader\Loader.exe) and select '' Run As Administrator ''

5. Follow the prompts and make sure you select the languages and applications you wish to be installed.
The Loader will backup and restore your data by default.
Still Having Issues !!?
I would recommend using BBSAK to Upgrade, so

- Uninstall the OS(s) you already have installed on your PC
- Delete the folder (research in motion) in ( c: \program files\common files\research in motion )
- Restart your PC

You cant do it at Desktop Manager our Telco OS are slow, you need to download and follow my guide above to update...

Remember to delete the Vendor.xml files. If still can't then uninstalled Desktop Manager and reinstalled again. Suspect the loader driver are missing that why can't detect. Which OS Windows you are using now?