The scheme is open to owners of Connect, Connect:Amp, ZP80, ZP90, ZP100, ZP120 and the first generation Play:5 for now. If you're one of those early adopters, you'll be able to follow an upgrade process through the Sonos app.
You'll see a Trade Up section in My Account, from which you can select eligible devices you'd like to trade in for credit. You'll get 30 percent off a new device for each one you upgrade, and 21 days later, your current speaker(s) will enter recycle mode. That'll scrub your data from the device and deactivate it permanently. You can then take it to your nearest e-recycling center to dispose of it or send it back to Sonos.