The bootloader is used to flash new firmware onto the Device, without having to use extra hardware like a PICkit3 . This makes it easier for end users to upgrade their devices.
The bootloader uses the Device Firmware Upgrade (DFU) protocol. This is a standard protocol for upgrading firmware over USB.
To update the bootloader you'll need special hardware and software that can interface with the PIC32 microcontroller.
The easiest way to get started is with a PicKit 3 programmer. You'll also need to install the MPLAB IPE software.