Install latest Blackberry Desktop manager go to Device>Update and Blackberry Desktop manager will check for update if it will not than there is a option "View other Versions" just click on it and select OS versions you want to upgrade or Downgrade.. :)
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Install latest Blackberry Desktop manager go to Device>Update and Blackberry Desktop manager will check for update if it will not than there is a option "View other Versions" just click on it and select OS versions you want to upgrade or Downgrade.. :)