Log in to your Sonos account online and check under Upgrade for your eligible products.

Once you have activated the upgrade credit it can then be applied to your basket.

https://www.sonos.com/en-gb/myaccount/user/offers/upgrade

You can also read the Upgrade T&Cs here https://www.sonos.com/en-gb/legal/upgrade-terms

Done that and found it thanks,all sorted