Are you iOS? you can’t, only on Android or Windows you can.

If you’re an Android user, head here https://en.softonic.com/download/sonos/android/post-download/v/16.1

You’ll need to allow third party installations through the settings on your device. Also remember to remove the new version before you try to install the old. Once you have successfully installed 16.1, make sure to disable automatic updates for the sonos app through Play Store.