Go into (Edit) the post or page that the image in on. Next right click on the image to open the image editor, then click the edit icon. After that the Image Details box opens. Their you can Add or Change the image caption, and the Alternate text.
Now if you look down to the Advanced Options and open it by clicking on the down arrow, you will see some more options.
* Image Title Attribute - putting text in the field will cause that text to appear any time you hover over the image.
* Image CSS Class - DON'T MESS WITH THIS
* Open link in a new tab - when checked, will cause the image when clicked on to open in a new window.
* Link Rel - Don't put anything in this field unless you know what you are doing. This information is put in the <head> tag as a meta tag.
* attachment wp-att-336 - leave it alone
* Link CSS Class - don't enter anything in this field. This messes with the CSS for link Rel tags.
Update your changes.
Then don't forget to update the page or post when fished with all your other changes.