改错：I don't know what to do it.

去掉it，或者改为about it也可以。。with it 我觉得不太恰当。

去掉并不会改变意思啊。。同样是我不知道做某事的意思。。。。what是指代it的，如果下文再用it，就会重复了

I don't know how to do it.或I don't know what to do .
都可以为：我不知道该怎样做。
I don't know what to do with it.意思是：我不知道怎样处理那件事。

去掉it
what to do
how to do it

I don't know what to do