<h2 class="widgettitle">Comic Chapters</h2>
This is the title of a widget.
Go to Admin Dashboard>Appearance>Widgets>review which one outputs this and manually add a title.
This may also be set in theme options.
If it's created by a theme template, consult with theme dev. That said, you can find all theme templates in the theme editor. Click a PHP file, place your cursor at top of editor, hit ctl and F at same time and search, repeat until found.
Edits made in the editor are not backed up, so make sure to back up any site files that may be edited before doing so...also review using a Child Theme.
Also, many computers can search within files, so download the theme and search that way also.