Learn how to Wipe/Erase OS on this BlackBerry device from here
Learn how to load BlackBerry OS after Wiping/Erasing from here
Follow the instructions on how to flash the device carefully from the link above
I will not take responsibility on whatever may happen to your device.
Make sure you are downloading the exact OS of the device you want to flash because
flashing a wrong OS, will make the device completely dead
(can't power on, can't charge and sometimes not be detected by a computer)
BLACKBERRY 9800 OS