Put the board into BOOTSEL mode: unplug it, hold BOOTSEL/BOOT, plug USB in, then release the button.
MCU: not detected
A standalone browser-based flasher. Choose a UF2 file, detect your Pico-family MCU, grant USB access and flash it directly from Chromium.
Put the board into BOOTSEL mode: unplug it, hold BOOTSEL/BOOT, plug USB in, then release the button.
Grant the browser access to the RP bootloader, then flash.
You can reconnect another board or flash again.