Q:

What is the correct version of "upfront", "up front" or "up-front"? 

I can't find a definite spelling of this term.  Different resources on the Internet show all three versions of it. 
What is the correct way of spelling it? 

A:

Let me say this up front: The NGram mplungian shows is pretty, but also irrelevant to your question.  First, it includes "be" in the phrase, which is not really the phrase you asked.  Second, Google NGrams has a bug which causes all hyphenated words to flatline (try it yourself, you'll see).  And we know that occurrences of "up-front" are not non-existent. 
Use "up front" when it's not a modifier:

Let's see if we can find a seat up front. 

But hyphenate it when it is:

Ralph had an up-front air about him.  You never had to dig to find out what was on his mind.