To edit the properties of an existing file, hover the cursor over the particular row that contains the file, when viewing the list of Media Library files and click the Edit link. Alternatively, you can also click the file title or the thumbnail image. Clicking the Edit link next to the file after it’s just been uploaded, will also allow you to edit its properties.

After clicking Edit, the file properties are displayed, including the title, URL, file name, file type and in the case of an image, the file dimensions. When editing the file properties, the Alternative Text field will only be displayed for images.

Title – Although the field isn’t labelled, the Title is the field at the very top of the page. The Title is displayed as a tool-tip when the mouse cursor hovers over the image in the browser.
Caption – is displayed underneath the image as a short description. How this displays will be dependent on the WordPress theme in use at the time. You can also include basic HTML in your captions.
Alternative Text – is displayed when the browser can’t render the image. It’s also used by screen readers for visually impaired users.
Description – This content is displayed on the file’s attachment page.