Is US English, 'ROUND' would not be involved: She crept up on him and smacked him on/in the back of his/the head. Agreed with other Dan: decide on neck or head. They're not the same thing.

Both have dodgy bits. Firstly we need to know: did she actually hit his head, or his neck? Because they are different body parts. Here's something I might say, if the scenario fits (which only you can say): She crept up BEHIND him and smacked HIM ROUND the back of THE head. But having said that... "Behind" is not strictly necessary here. "On" is fine, if you want to be a bit vague & slang / hyper-casual. The second "Him" is quite important. A good grammar improvement, to make the sentence complete. "Round" is good for being more natural here, but not very important. Changing "his" to "the" near the end, was a very small, unimportant nuance for smoother flow in casual talk, simply based on how commonly people use "the" this way in similar phrases.