Your website is built using WordPress, a free, open-source, website platform used by thousands of people worldwide. WordPress is actively developed and maintained, so updates are frequent, and new features are added all the time. This also means that support resources and documentation for WordPress are numerous and easy to find in comparison with other platforms.
The main documentation site for WordPress is called the Codex, it can be found here https://codex.wordpress.org
Although the Codex caters for advanced users and developers, it also has beginner-level guides relating to editing content. You can start here https://codex.wordpress.org/WordPress_Lessons and browse the pages. Please bear in mind that every WordPress site is a little different, and features on your site may not be part of the ‘core’ WordPress – they may be created using plugins, or bespoke code, in which case they not be covered by the Codex explicitly.