If you turn the remote control upside down and turn so the battery latch is facing away from you, you will notice a little latch on the right hand side - move this to the left towards the latch and pull the latch out. It may be a little stiff as it's a tight fit but you should be able to slide it out with a bit of elbow grease. The battery is a 3V lithium cell battery which should be available from any well stocked local electronic components supplier.