*** YOU NEED TO REGISTER ON THE FORUM BEFORE YOU CAN ACCESS THE DOWNLOAD LINK ***
Once you have registered you will be able to download the firmware by clicking on the orange "DOWNLOAD NOW" button.
Once you open the link, you will need the decryption key to access the firmware - hit the 'spoiler' below to show the key.
!spZV3a9hMjNil0Lzq8H5RmnFW4gZKfO6959i6sB7EQw
INSTALLATION INSTRUCTIONS
http://www.xperiablog.net/2016/03/12/install-xperia-marshmallow-using-flashtool-and-xperifirm-guide/
- Download the FTF firmware file
- Ensure all drivers are correctly installed on your computer as highlighted in Part 1 of the guide linked to above
- Follow Part 3 of the guide to flash the firmware to your Sony Xperia device
XPERIA FIRMWARE DETAILS
This is official stock Sony Xperia firmware.
Model: Sony Xperia C4 (E5306)
Firmware version: 27.3.A.0.165_R1B
Region: US (Generic) [1295-8051]
Android version: 6.0

Xperia C4_E5306_27.3.A.0.165_US_Generic 6.0
Sony Xperia C4 (E5306) Android 6.0 firmware - Generic US