The name of the Bangle changes from "Bangle xxxx" to "DfuTarg" when it is in the DFU mode. It is what you need to select in the app by Nordic.
Bangle.js 2
Long-press the button for about 10 seconds until the screen goes blank
While ==== is going across the screen, release the button (releasing it later will just boot back to Bangle.js)
The watch should now be in DFU mode
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
The name of the Bangle changes from "Bangle xxxx" to "DfuTarg" when it is in the DFU mode. It is what you need to select in the app by Nordic.
You can find the complete description here:
https://www.espruino.com/Firmware+Update#android-ios-app