Before disconnecting battery, review Soldering Tips to ensure proper soldering.

Use soldering iron to remove +(red) battery lead.

Touch the positive wire to a metal surface acting as a ground to discharge the battery. Depending on the charge level of the battery, it may take up to 30 seconds to insure that it is fully discharged.

'Ensure that the wires and solder do not touch each other, for this will damage the device

Remove the -(black) battery lead.

Remove and discard the old battery. Insert new battery and solder the positive and then the negative terminals to the circuit board.