You don't say what the original Chapter One looks like in code view. Just the text isn't sufficient to make sure the find/replace is correct.
Assuming you have
Code:
<p>Chapter SOMETHING</p>
and want
Code:
<h1>Chapter SOMETHING</h1>
then
Code:
Find: (?sU)<p>Chapter (.*)</p>
Replace: <h1>Chapter \1</h1>
might get you what you want.