How to add or edit featured image or post thumbnail
Featured images, also known as post thumbnails, are a prominent element in many WordPress themes. They typically display above or alongside the brief summary of a post. These images play a crucial role in various features of our site, like the related posts carousel, widgets for recent or popular posts, and thumbnails for related posts. Essentially, they are the primary images representing the posts and are used throughout the site for visual representation.
How to add a featured image or post thumbnail

On your Post Editor click the Settings Icon. Then click the Post Tab.
Scroll down and click the Featured Image tab.
Click the Set Featured Image button, and upload your post thumbnail or you can pick an existing image for your Image Gallery.
Click the Update or Publish button to save your post.
Tips: Using a plugin to manage your featured images
I love using Quick Featured Images plugin to manage my Featured Images. It helps me change and assign my post thumbnails quickly and easily.