Follow the instructions on this site and you should be fine
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I would recommend making a backup of your data using backup/restore before attempting this, even though the application loader automatically backs up your data. Many times I have seen people loose their data because during the loading operation something goes wrong, and they didn't take the 2 minutes or so to make a backup.
Also, if you are on a BES, you may have to get your IT people to set a EA password if your EA does not kick off automatically, of if you use BIS, you will need to login to your carrier's BIS site and resend your service books.