Update your 3D printer firmware without a bootloader!

I also explain how to burn a bootloader to a 3D Printer that doesn't have one yet. But really, you might not even need one!
Use a USBtinyISP as a programmer or re-purpose an Arduino board you might already have to upload anything even to mainboards that do not come with a bootloader installed.

USBtinyISP drivers (Windows) https://learn.adafruit.com/usbtinyisp/download

Get a USBtinyISP
Adafruit https://www.adafruit.com/product/46
Aliexpress http://s.click.aliexpress.com/e/ctRyQ4Sy (get one with the cables)

Cheap Arduino boards
Arduino Nano http://s.click.aliexpress.com/e/2PwETWC
Arduino Uno http://s.click.aliexpress.com/e/b52ipsmM