The text overlay is the “caption” and “description” for the image. There are to possibilities to change it: 1. Search the image in your media library ( WP Admin -> Media ), edit it and set “caption” and “description” 2. Recommended! Edit the page with the “Image-header”, then click on the “Featured Image” box to the right on the image, then in the pop-up window click the “Media Library” link on top and set the “caption” and “description” for the image. Then close the pop-up window and save the page/post. 
