The transformer has a pcb board with a fuse for the main input. If you want to change the voltage , you must first remove the pcb board.
The main lead is connected to the 0V and 115V as show in the photo.
115 Volt: Remove the wire between the mid 0V -115V. Connect the not connected 0V to the other 0V and the not connected 115v to the other 115V.
230 Volt: connect the mid 0V to the mid 115v. See Photo