The firmware consists of 2 parts:
The touchscreen firmware and the printer firmware.
You need to flash both! One will not work correctly without the other!

Requirements:
- microSD card (8GB or smaller)
- hex screw drivers (come with the printer)

Download the latest release of the firmware from here.
In the downloaded zip file you will find 2 folders named "Marlin" and "LCDFirmware" and a file named "i3PlusPlus_XX.hex"

Installing the printer firmware:

-Using Cura:
Under Machine add your i3 Plus as a Machine.
Then click Machine again and press "Install custom firmware".
Select the "i3PlusPlus_XX.hex" and hit open.

-Using Arduino IDE:
Connect your printer to your PC.
Download the Arduino IDE from here.
Open the Arduino IDE and under Tools select Arduino/Genuino Mega or Mega 2560 as your Board.
Make sure ATMega2560 (Mega 2560) is selected as a Processor.
Select the COM-Port your printer uses under Port.

Click File->Open and select the Marlin.ino located in the Marlin folder in the download.
Then click the Upload button which is repesented by a right pointing arrow.
When the Upload is finished your printer will reboot.

Installing the touchscreen firmware:

Get your microSD card and insert it to your pc. Right click it and choose format.
Choose FAT-32 for file system and 4096 Bytes for sector size.
Also uncheck quick format.
After formating finished, open it up and copy all contents of the LCDFirmware directory to it.
Then insert the microSD card in your touchscreen and power on the printer.
You should see changing images on your touchscreen.
The screen will stay white when the flashing finished.
Remove the microSD card and reboot the printer.
After that reboot your printer and you should see the new boot animation.

Look here if you dont know how to access the touchscreens microSD slot.