From the Dashboard of your WordPress website, go to Media > Library
You will see all of your images there. Select the image which one you want to update, and click “Edit”.
The “Edit Media” page will open. Scroll down to the bottom of this page, you will be found few input boxes to provide the Caption, Alternative Text and the Description of your image (like below screenshot).
Please make sure, you have included the alt attribute in your theme.
