Older versions of the nRF52840 were shipped with a bootloader that does not handle large UF2's, and has other issues. To check whether you need to update the bootloader, double-click the reset button, and look in the NRF52BOOT drive for INFO_UF2.TXT. Inside that file, check the version number. It should be 0.6.1 or newer. This one is older:

UF2 Bootloader 0.2.9 lib/nrfx (v1.1.0-1-g096e770) lib/tinyusb (legacy-755-g55874813) s140 6.1.1
Model: Adafruit Feather nRF52840 Express
Board-ID: nRF52840-Feather-revD
Bootloader: s140 6.1.1
Date: Feb 22 2019

UF2 Bootloader 0.2.9 lib/nrfx (v1.1.0-1-g096e770) lib/tinyusb (legacy-755-g55874813) s140 6.1.1
Model: Adafruit Feather nRF52840 Express
Board-ID: nRF52840-Feather-revD
Bootloader: s140 6.1.1
Date: Feb 22 2019

Adafruit nRF52 Bootloader can be upgraded/downgraded without any additional hardware. There are 3 different ways to update bootloader, each has its pros and cons:

Use UF2: 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).
Use Arduino IDE: work with all Adafruit nRF52 boards and bootloader version, typo free but may not be the latest bootloader version due to BSP release cycle.
Use Command Line: work with all boards and bootloader version (including 3rd party one). This command line is back-end of the Arduino IDE method above.