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.
You need a .
uf2 file containing the latest version of the bootloader. Use the link below to download it.
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.
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.