This page covers how to reflash the bootloader on a Fly-CDYv3.
Unfortunately, it is not possible to enter DFU mode on this board through USB.
This means that you will need an ST-Link to be able to reflash the bootloader. Fortunately these have been cloned by chinese manufacturers and can be picked up cheaply.
These instructions cover flashing with Windows using the STM32CubeProgrammer software. It is possible to use other software, such as DFU-Utils on linux, but they aren’t covered here. You will have to register to be able to download the software.
Connect the ST-Link to the Fly-CDYv3 using 5 Jumpers. Connect the following:
Launch the STM32CubeProgrammer Software.
Download the bootloader from here.
Select the correct connection method, in this instance its “stlink”. You may need to click the refesh button to find it. Then click connect.
Then click on the “+” icon and find/open the bootloader file you downloaded earlier.
Click the “Download” button.
A “File Download Complete” popup will appear once complete.
Disconnect the board and continue flashing the firmware as normal.