First of all, back up everything as best you can. Then obtain a BR2330 or CR2330. These are Panasonic numbers, other manufacturers will have equivalents.
If you want to change the battery without losing information, it gets more difficult. If you are not a skilled and experienced technician, stop reading immediately and find someone who is (a dealer's Service Dept would be a place to start). If you are suitably qualified, you will not need further instruction.
Stored information can only be preserved by opening the unit and replacing the battery with the power connected. THIS EXPOSES YOU TO POTENTIAL DANGEROUS VOLTAGES AND IS NOT RECOMMENDED AND MUST BE PERFORMED ENTIRELY AT YOUR OWN RISK.
You get the picture – do a decent backup to a reliable card AND a computer, then you should be able to reinit and reinstall after changing the battery. If for some unknown reason it doesn't work, you'll just have to recreate the settings.