My charge 5 recently stopped working for no apparent reason. I contacted customer support, tried everything they suggested and was told the only solution was a new device. I just read an article that this has happened to a lot of people recently. If that's true, Fitbit needs to identify the problem and pay for our new devices. Seems very suspicious.
To upgrade, open your Fitbit App. Tap the battery icon in the top left. Tap Add more Devices. And follow the prompts
With android, once the Charge 6 powers up., in a minute you should see a popup asking to install the Charge 6