HubSpot's drag-and-drop editor should be easy and intuitive to use. For the most part, you should be able to click anywhere on the page to edit, rearrange, add or delete content.
Follow the instructions here to create a page using one of your theme's templates. Within the page you have several elements:
Section: this is the container that wraps your content. To add one, hover over the bottom of any section and a "+" sign will appear - from there you can choose one of the theme's prebuilt sections or a layout. You can also save sections you like for future use. When you have your section, you can edit its background (image, colour, gradient...), spacing and alignment as well as drag content into it.
Columns: within your sections, columns allow you to arrange your content vertically.
Rows: within your sections and columns, rows allow you to arrange your content horizontally.
Modules: while all of the above is mostly structural, modules are where you actually add your content such as text and images. HubSpot includes several default modules such as Rich Text, Image, Form... etc. and your theme includes several custom modules such as Accordion, Blog Posts, Custom Button... etc.
For more detailed tutorials from HubSpot check out this article.
If you're new to editing on HubSpot, I'd also recommend taking time to review HubSpot's CMS for Marketers certification - in particular the "Creating Pages in the HubSpot CMS" section.