First of all start the application and select the image. By the default Photo Exif Manager opens the Pictures folder. You can change the folder in the tree view at the left bottom of the application. Then in the right bottom corner select the file. After what, it will load image to the preview area and will load exif tags to the tags list.
To edit tag, select it in the tag list.
Then you can use mouse double click, menu "Exif Tag" -> "Edit" or button "Edit Tag" on the Tool Bar.
Correct the tag value and click Ok button.
Edited tag becomes highlighted in bold. To commit the changes you need save them, use Save button on the Tool Bar or use menu "File" -> "Save". To save image to a different file click the "Save As" button on the Tool Bar or use menu "File" -> "Save As".