A WordPress page is normally constructed of three or more parts, each editable in different ways.
The most common part that you will edit is the "Content" section of the web page.
Each page on a website represents a different Content section, but other elements are reused on every page.
Normally these reusable section are set up when the website is created and never need to be changed after that. If you do need to change anything you will find options for these:-
The logo can normally be changed under Appearance / Customize / Site Identity.
Menu items can be changed under Appearance / Menus.
Not all websites have a sidebar, or they only show on certain pages.
They are normally comprised of "Widgets" which can be edited under Appearance / Widgets.
Sometimes these will contain "Widgets" which can be edited under Appearance / Widgets.
Next: Find the Edit Page