Have you seen/followed the guide?

https://support.sonos.com/article/connect-sonos-to-a-new-router-or-wi-fi-network

Yes. There is no option to “update network”. It just says your system was not found on “new network name”. Set up a new system!!!

It’s truly rubbish that this isn’t a simple thing. I’ve now get the system working but on my old WiFi network. I therefore need to change it so it’s on my new network but there is no way of doing this!!!

I change ISP providers and/or routers at any time I like. The simplest way to do so is to keep the same SSID and Password.

• Prior to turning off your current router and/or completely changing the router/modem (if provided new ones by the ISP) unplug all your Sonos.
• Once the new ISP is connected keep the same SSID and password.
• If using equipment (router/modem) provided by the new ISP have them set the router to use the same SSID and Password as was used before the change.
• Make sure your device with the Sonos app is connected to your network (It should be if you have used the same SSID and password)
• Plug in your Sonos and they should connect with no issue. 

What you must understand is that the Sonos gear is only looking for a WiFi signal with the same SSID and Password. Sonos gear doesn’t care what router is sending the WiFi nor who is the ISP.

Even easier than using the same SSID/password is to bring your router with you when you move! That way absolutely nothing has to be set up in your new location, and no devices need any changes.