If you use
Sigil, you can use Regular Expression search and replace.
The regular expression would be:
Search for <p class="chapnum">(.*)</p>
Replace with <h1>\1</h1>
Be sure to select minimal match and do the replace in all html files.
You probably also want to create a style for you header to have it look the way you want. Something like
Code:
h1 {
margin-bottom: 2em;
margin-left: 0;
margin-right: 0;
margin-top: 10%;
text-align: center;
text-indent: 0;
}
There is a whole thread on this forum about Sigil so if you have specific questions about using the software, you can ask there.
Best