Polar ft7 battery changing

Sondrasr

Does anyone have any suggestions on how to change the battery on my polar ft7. I tried to use a coin in the back of the transmitter but it doesn't seem to move as well as all it seems to do is take the plastic off the sides of the coin slot. I have tried all side coins as well as a flathead screwdriver.
Does anyone have any suggestions besides sending it back to polar.
Thanks everyone.