1. Download the file onto a computer
Then transfer it onto a USB drive or SD card
Then insert into box

2. Use reset button inside AV hole on the back of your 3900 to get into recovery mode on first time power up.
You will need to hold the Reset button on first time power until you get into Recovery Mode.

You will need to use a Tooth Pick or Paper Clip or something similar to do this.

3.
Once in recovery
Go to
Apply update from ext

Select Udisk which is the USB Drive or SD Card depending on which one you have the Firmware stored on

4.Highlight the Firmware
Press Ok to install it

It will take up to 8 mins to install

5. Once finished
Select reboot

6. It will boot back into Android and everything is normal and nothing is erased.

Code:
https://mega.nz/#!vUtgnIrJ!xD0px7qHy68TrZlxEv9yE47hZyexYVXPkjuhD5bn0fs