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 in 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.