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To upgrade your BlackBerry to the latest OS version, you have to go to the provider-specific download page::
The most recent BlackBerry Desktop software is available here: http://www.blackberry.com/support/index.shtml
In most cases it is also possible to install an update from a different carrier (if they have the same BlackBerry model). This might be useful if your carrier does not provide the most recent version you need, or you want to have a special foreign charset. For example you can install the “Hong Kong CSL” version to have chinese character and input support (the Operation System is still in english).
To install a OS update from a different carrier you have to delete C:\Program Files\Common Files\Research In Motion\AppLoader\VENDOR.XML first.