For instructions on the Replace function, see
http://www.gmayor.com/replace_using_wildcards.htm
For this particular issue, with the wildcard option set search for
Chapter [0-9]{1,}^p
and replace with
^&
and with the cursor in the Replace with box, click Format. Now what you do here depends on how your document is already formatted.
The last thing you want to do is to add empty paragraphs. You could add Paragraph Format > Space After of (say) 24 points, which will increase the space after the Chapter headings, or better still you could create or modify a style for your Chapter headings complete with Space After and use the replace Format function to apply that style.
Styles make reformatting a much more pleasurable task than struggling with manual formatting.