both of them mean "how" 如何is more formal than 怎么样,but the meanings are the same 这个看起来如何？=这个看起来怎么样？ people usually use 如何 in writing or formal situation but there is an exception you can say 你怎么样了(means how are you)？ you cant say你如何了？（this sounds a little odd）

如何:it tends to be used in ancient chinese or in written chinese ，怎么样: it likes a oral one,you can hear 怎么样 in normal mandarin at street or 胡同,such as how are you.,what is up. in brief,如何 is formal one，but 怎么样 is more oral or informal

如何is often used in literal, but 怎么样 is focus on oral.

如何和怎么样的相同点在用于句尾,表示询问状况或征求意见时,两个词可以互换.例如"我想买这本书,你觉得如何/怎么样?" 如何和怎么样的区别:怎么样可以用于否定句.如何一般不用于否定句.例如"今天的天气不怎么（=不好)" "这本书不怎么样（=不好)"

意思基本上一样 只是“如何”比较书面语化