What is the difference between 怎么 and 如何？ Is it mostly a question of register?
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: 


  如何？


There is an idiom 如何是好, which expresses "What/How should I do?".