Hi. The Mobicel Onyx's firmware has yet to be made available online unfortunately. If you are still under the one year warranty and still have the receipt, then take it via your retailer to avail an in-warranty software reload service. Otherwise, you can resort to the option here http://satechhelp.co.za/9877/Return-Mobicel-Phone-Tablet-Firmware-Flashing
If for some reasons, non of the preceding options are convenient for you, then you could also take the Mobicel Onyx's back cover > note down the system board's ID and share that with us. Checking with your local phone repairs shops is yet another possible recourse.