1. Deleting the certificate manually from security settings.
2. Getting your phone flashed at an unauthorized repair shop. It is not recommended to do so for it voids the warranty, it deletes all the certificates from the phone and you may end up with a dead device.
1. Update your software using Firmware Over The Air (FOTA). If there is any update available, certificates will be sent on your phone - download & then install them.
2. Try re-installing the firmware using Nokia Suite/Nokia PC Suite. It will install the genuine software for the phone and along with it the certificates will get installed.
3. If nothing works please contact an authorized Nokia Care Center that is near you. You can find the location at Nokia Support.
For firmware re-installation/update (Guide & Things to Remember):
Connect your phone to a computer & select Nokia Suite mode. If you do not have Nokia Suite on your PC you can download it from here. Now open the Nokia Suite & click on the “green icon of arrow pointing downwards”. If any updates are available they will be shown in ‘green’. If not you may re-install the current version. Re-install option is shown in ‘blue’. If no option to re-install is shown, please wait for sometime it will become visible. Then the Nokia Suite will download the new software version of the phone. After it you will be prompted to take a complete back up of the device content. Please do not skip this step as re-installation will erase all the personal data from the phone like contacts, messages, calendar entries etc. Please note that if you have set a memory card password & you have forgotten its code then take a backup of the card data also as the card will be locked after update and can only be unlocked if you enter the correct code or by ways mentioned in this thread. Also note that while updating the device ensure the following things-
- The device is full charged.
- There is adequate free space in internal memory of the phone. Move all the content that you can move from the phone memory to the memory card. It will free up precious space in the phone memory which will decrease the chance of failure of update.
- The internet connection is of good speed & has a backup in case of power loss.
- The PC should also be connected to back up power source as if the process is interrupted the phone may go dead. More on update failure & its remedies here.
- Please do not run any resource intensive programs in the background (on the PC), while updating, as it may freeze the PC which in turn will lead to a dead device.
- Ignore all the windows pop-up notifications about device being disconnected.DO NOT DISCONNECT THE PHONE UNTIL UPDATE IS COMPLETED.
- It will be better if you disable security key-guard & PIN code request before proceeding with Update/Re-installation.
- Sometimes the device may show Test Mode. Do not panic & DO NOT remove the cable. Let the process complete. Hope this helps.