The scheme is open to owners of Connect, Connect: Amp, ZP80, ZP90, ZP100, ZP120 and first generation Play: 5 for now. If you're one of those first adopters, you'll be able to follow the upgrade process through the Sonos app.
See you see the Trading in My Account section, from which you can choose devices that meet the conditions for which you want to trade for a loan. You'll get a 30% discount on a new device for everyone you upgrade to, and 21 days later, your current speaker (s) will go into recycling mode. This will erase your data from the device and permanently deactivate it. You can then take it to the nearest e-recycling center to use it or send it back to Sonos.