If you already have your board in Bootloader Mode and you have MicroPython (Raspberry Pi Pico) selected, it will automatically come up with the installation screen. If it does not, follow the steps below to install.
While holding the BOOTSEL button, plug in your Pico. It should show up under the Target device fields.
If the board is new out of the package, you do not need to hold BOOTSEL while plugging it in.
Click the Install button to begin installation. It will take a few seconds for it to copy the firmware onto the board.
If it comes up with a message that it couldn't find the device, try hitting the Stop/Restart button at the top a few times.
You can always select your port manually. Select the your port from the Dropdown menu. These will vary from system to system.
If it comes up with an error that the device is busy, try clicking on the Stop/Restart Button at the top.