从句意看，how 和 that，只能保留其中之一。how 描述方式；that 陈述事实。

I already have declared as emphatically as I know, that the quality of human life in our society depends on the quality of the schooling we give our young people.

【翻译】我已经尽我所知强调指出，我们社会中人的生活质量取决于我们向年轻人提供的教育质量。

I already have declared as emphatically as I know, how the quality of human life in our society depends on the quality of the schooling we give our young people.

【翻译】我已经尽我所知强调指出，我们社会中人的生活质量如何取决于我们向年轻人提供的教育质量。