Use this,
https://www.singaporeair.com/feedback-form/

They usually respond in 5-7 days.

Im not agreeing with their practice of changing seats without consultation.
I too have bulkhead seats (non-bassinet) changed once some years ago; and it wasnt for a family/infant. Apparently someone with a higher status than me.
I insisted firmly till they reverse that change.
Its absurd especially when they do it without consultation, or at least offer a change that is fair.