This is the fastest and safest way to update bootloader. However, it requires your existing bootloader is at least 0.4.0 and only work with nRF52840 (not nRF52832) since UF2 make use of USB MSC interface.

Download update-bootloader UF2

You need a .uf2 file containing the latest version of the bootloader. The most common updaters are linked below. For updaters for other boards, look for the .uf2 files here that begin with update-...

Enter Bootloader Mode

Double-click the Reset button on your board, and you will see the NeoPixel RGB LED turn green (identified by the arrow in the image). If it turns red, check the USB cable, try another USB port, etc.

Note: on nRF52840 USB Key with TinyUF2 (PID 5199) you need to hold its button while plugging into your PC.

Drap & Drop UF2

You will see a new BOOT disk drive appear e.g FTHR840BOOT. Drag the downloaded .uf2 file to FTHR840BOOT. The LED will flash. Then, the FTHR840BOOT drive will disappear. That's it, you have successfully update your board to the latest version.   

Issues

If you download the wrong UF2 file for your board, dragging the file to the ...BOOT drive will close the drive but will not update the firmware (as it is the firmware for the wrong board). You can check the INFO_UF2.TXT on the board and see if the firmware was updated (likely with the wrong UF2 file it was not). 

If you cannot find the correct UF2 check all the files in https://github.com/adafruit/Adafruit_nRF52_Bootloader/releases for the correct UF2 (they all start with the word update-boardname-bootloader-version.uf2 so scroll the list down to the files starting with u.

Non Adafruit board firmware updates can be found using this method also.