After you successfully removed the PCB from the case, the next step is to cut the cable tie that is holding the old lithium battery onto the PCB using a wire cutter.

Next you will have to remove the lithium battery from the PCB, by cutting its two terminals that are soldered onto the PCB, with a wire cutter.

As an alternative, you also can directly desolder the lithium battery, without cutting its terminals.

Make sure not to touch the upper part of the PCB or the geiger tube, because these parts of the device are under high voltage, until the battery is removed.