Pick the board you have, click Download UF2, then put the board in bootloader mode by
double-tapping the reset button on the board (within ~500 ms). It will mount as a USB drive — drag the downloaded .uf2 onto that drive. When the drive disappears, the board has rebooted into the new firmware.
firmware.zip over Web Serial DFU
The transport CI does not currently publish DFU .zip packages, only UF2. If you have built one locally, upload it here. Web Serial DFU avoids the drag-and-drop step but is otherwise equivalent.
After flashing, connect to the running firmware to read and edit the persistent config. USB and Bluetooth speak the same msgpack protocol — pick whichever is convenient.