First method:

1. Press Power button until your cell phone turns on.
   
2. Then press and hold * key + 3 key + Call button + Power button.
   
3. When you see nokia logo, then release holded buttons.
4. Now enter your lock code.
   * The default lock code is 12345
5. After that your phone will reset and restart.
6. That' all.

Second method:

1. Switch on your handset.
   
2. Tap this secret code: *#7370#
3. Next please press your lock code.
   * The default lock code is 12345
4. Now your phone will reset.
5. Success!

Third method:

1. First turn on your phone.
2. After that press Menu button and goto Settings -> Phone -> Phone management -> Factory Settings ->Delete data and restore.
3. Confirm your decision -> press Yes.
   
4. That's all.