Dear Andrey,

Editing the theme is fairly easy: you have the portfolio items and normal pages with options and content. The most important portfolio options are the hero image and captions (title and subtitles). The content can be edited as for normal  pages, nothing special about it.

So, to answer your question you edit the hero title (the one from the screenshot is called hero title) by using the page and portfolio item options. You access the options panel when you Edit the page/portfolio item and scroll down past the editor. There is a Hero Section tab where you can set the hero image, hero title and hero subtitle.

Scroll down over de editor until you see the page options panel (or portfolio options if you are inside a portfolio item)

If you still cannot find it, send us wp admin access in a private comment we can have a look