You will need to telephone Sonos as this is the only way old system can be updated.

If indeed the controller process for updating is failing, you do need to contact Sonos support, they’ll put together a special “update” for you, and point you to it so that you can update your product. 

In general, though, you should be able to update using the Sonos controller, by going to Settings>System>System Updates>Check for Updates.