Replace index.html content with proxy like charles or fiddler, which is visited by BoseUpdater.
The index.html's URL should in https://worldwide.bose.com/connected_device, as in China, it's https://www.bose.cn/ced/bose_music_china/lando/index.xml
Press 'A'-'D'-'V'-'Up'-'Down', then you will see the version select box to apply version update.
Check BoseUpdater.log in your macOS or Windows, it will tell everything during DOWNGRADING.
I downgraded my QuietComfort Earbuds from 2.0.7 to 1.0.7 successfully, but the cancelling ability is not back to perfect as I first time enjoy it after bought from store.
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The downgrade operation is in high RISK. Please don't do it if you don't know what will happend.
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