BBSAK (BlackBerry Swiss Army Knife) is a very good tool you can use to wipe and load a new operating system on your blackberry phone.

REQUIREMENTS

1. BBSAK
2. .NET Framework
3. Desktop Manager
GUIDE:
1. Always backup your phone. Backup your phone using Desktop Manager.
2. You can backup your 3rd party apps too if you want but I usually don't. I will do a fresh download of those apps to be on the safe side.
3. Plug in the cable to your phone and run BBSAK.
4. You will see a box to enter the password. If you don't have a password, leave it blank and press OK. If you have a password, key in the password and press OK.
5. You will be able to see your BlackBerry PIN at the PIN box if the phone is connected correctly to BBSAK.
6. Search for the option to Wipe Device and it will prompt you if you are sure to wipe. If you have not backup your phone (or forgotten to), this is a good time to exit and backup first.
7. Press Wipe Device and wait until the box appear saying Wipe Complete.
8. The phone will reboot and you will then proceed to a white screen. It will prompt you Error 507. Do not panic and follow the next step and don't unplug the cable.
9. If you have the desired OS installed in your computer, select the option for Load OS.
10. From here, everything is straight forward and just follow the instructions.

Source: http://www.michaelokeje.com/2012/04/how-to-wipe-and-load-os-on-your.html