In this sentence, the use of *how* is exactly parallel to some uses of *whom*.

> He told us how to do it.

> He told us whom to see about it.

However, in these uses, *whom* is officially classified as a [relative pronoun](http://en.wikipedia.org/wiki/Relative_pronoun), and *how* as a [relative adverb](http://simple.wiktionary.org/wiki/Category:Relative_adverbs) (although sometimes it is called a [subordinating conjunction](http://en.wikipedia.org/wiki/Conjunction_%28grammar%29#Subordinating_conjunctions)). It makes you wonder whether the definitions of all these parts of speech were designed with a different language in mind, and the grammarians are shoehorning English into them as best they can.