Is it mostly a question of register?
- Anybody can ask a question
- Anybody can answer
- The best answers are voted up and rise to the top
Normally, 怎么 is more oral than 如何. 如何 can be read in much ancient literature, and it can also be used in formal documents, but that is too formal.
You may express "How about that?" saying:
There is an idiom 如何是好, which expresses "What/How should I do?".
Actually, I believe both "怎么" and "如何" have multiple senses. So you'd better to look up a thesaurus or specify which sense you are referring here.