Is it correct to say, "Please feel free to change anything and everything in the draft"? I want to mean the reviewer can change as much as he wants (but want to say that more emphatically).
What would be a better expression?
You are probably fine using that phrasing, but you might want to clarify your meaning further (so it doesn't sound like you are asking him to literally completely rewrite the draft). For example, you could say "Please feel free to change anything and everything that needs revision." Alternatively you can say something like "Feel free to make any changes you deem appropriate."
Side-note: I find it mildly humorous that the idiom "anything and everything" generally means neither.