Mouse over the Photo module to be edited and click the spanner icon to open the editor. Your Photo module has already had custom styles set. This means that you can replace the photo and all your custom styles will be applied automatically.
On the General tab, click Edit or Remove as shown below. This will open the Media Library where you can upload a new image or select an alternative existing image. Set the image to be shown at Full Size, unless specified otherwise.
Save and close!