The easiest explanation is that 如何 is literary Chinese, classier and less colloquial than 怎么.

1. They are almost the same in most cases! 如何才能追到你？ <-- equals to --> 怎么才能追到你？ 2. But they're not the same when they're used as independent sentences. 我们去看电影吧，如何？ "如何" is equal to "怎么样" here. 怎么？你不高兴？ "怎么" is short for "怎么了" here.

usually it is same in chinese. 如何，we usually use 如何 in an article or a book it's a little bit offcial and 怎么 is much more colloquial