It probably unscrews, but it looks like you have enough of the locknut exposed so that you can remove it and the stem without removing the flange. The handle and stem look like Valley, but since they were an imitation of Delta, the two are outwardly similar. Internally, however they are "opposites", (although you can interchange them), and if you use the wrong stem, your lever handle will work backwards. Your lever handle will either point the wrong way when closed, or it will rotate in the opposite direction when you turn it on.