I don’t know how to do it.
这一句中用how,而不能用what,理由是what是疑问代词,而how是疑问副词.
how 表示如何,怎么样,在句中作状语.
what 表示什么,在句中可以做主语,宾语,表语.
例如：
I don't know what to do.
I don't know how to do it.
How do you finish your work?
What do you see in the picture?