Newly stocked versions of the headphones will come with Alexa support already (as signified by a sticker on the box), but those who already own a pair will need to update the firmware.
This is simple. Just connect your headphones to a PC or Mac via USB and visit Bose's specific device updater website here: BTU.Bose.com.
It will take you through the steps required to update your QC35 II headphones to the latest firmware, v3.1.8.
Amazon also updated its iOS and Android versions of the Alexa app yesterday, 7 August, in order to make it compatible. You should now be able to ask Alexa questions by pressing the action button on the cans.
Bose also added Google Assistant support to the 'phones in recent times so you can choose which voice assistant to opt for.
You need to assign which one to use through the latest version of the Bose Connect app for iOS or Android.
As well as Alexa, the new QC35 II firmware addresses some stability issues, big fixes and features some performance improvements.
Also see: Bose QuietComfort 35 II Headphones UK Review