Solder a new battery to the middle part of the wires on the original battery. Then, clip the old battery out above those solder connections. Do NOT remove the old battery first! Just use the same voltage battery. Lithium type preferred.
The unit id will hold for about 15~ seconds on loss of power, but I wouldn't trust that as it's a pure gamble. Leave the old battery in until the new one is soldered in parallel as I said above.