You would need to run a repair to reload the firmware, please be aware that this will delete all user data except pictures, music and videos:
To repair the phone software
1. Install PC Companion on a Windows PC from Support at Download for PC - Sony Smartphones (Global UK English)
2. In PC Companion, under Support Zone and click Start.
3. Under Phone Software Update, click Start.
4. Click repair phone.
5. Follow the on-screen instructions.