In order to add new pages or edit the existing ones in Magento, you should go to the Content menu and click on the Pages option under Elements section.
Magento 2 comes with 4 default pages but if you want to create a new one, you should click on the Add New page button.
Clicking that button would redirect you to the new page addition option where you may add Page Title, Content Heading, Content, SEO options, Theme, etc.
When you fill in the desired option - just click on the Save page button and that's it - you have added the new page.
In case you want to edit an already existing page, again go to the Content menu and click on the Pages option under Elements section. Now just choose the page you want to edit and click on it to preview the main editing options such as Title, URL key, Status, etc.
In case you want to edit other part of the page such as its content, you should expand the Select menu and click on the Edit button from the drop down menu.