How to prepare your Nano V1 for updating

Make sure the power switch is OFF and the USB cable is disconnected. Hold down the small button on your Nano with one hand, and with the other hand connect the USB cable to your PC or laptop. Your Nano is now in boot mode and ready to receive firmware.

The status LED will be off — that's normal in boot mode. The charging LED may still light up, which is fine and just means the USB connection is good.

Tip: It can be easier to plug the cable into the Nano first, then hold the button while you connect the other end to your computer.

Web-based firmware updates require Web Serial, which is supported only in desktop Chrome, Edge, Opera, and other Chromium-based browsers. Firefox and Safari will not work.
Mobile browsers probably are not supported either. You must use a PC or laptop.

Many USB-C cables, especially short ones bundled with phone chargers, are wired for power only and have no data lines. These will charge your device but cannot communicate with it, so the updater will not be able to detect your device.
If you have problems connecting to your device this is probably why.