Changing Images

There is a change column to the left on the images section of Products. Can you only use this to change text or can you substitute new images? I want to replace the top image but not the other 20 images

Indeed, it's only used to change the name. If you want to change the image via the interface you need to delete all the images and insert them again.
Otherwise, just go by FTP in the folder media/com_hikashop/upload, replace the image there by the new one (you need to have the same name of file) and also go in the thumbnails folder and delete the thumbnail which was generated automatically (that way, hikashop will automatically generate the new thumbnail).