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If your mobile service provider has approved and released 7.1 to you, then you can do it OTA (though, honestly, I highly recommend AGAINST that method). But, if they have not, then you must use a different carriers OS package, and doing so can only be done via PC (not even MAC) and not OTA.
Occam's Razor nearly always applies when troubleshooting technology issues!
If anyone has been helpful to you, please show your appreciation by clicking the button inside of their post. Please click here and read, along with the threads to which it links, for helpful information to guide you as you proceed. I always recommend that you treat your BlackBerry like any other computing device, including using a regular backup schedule...click here for an article with instructions.