its a very small battery something to the direction of LR44 most likely even smaller and rechargeable.
You will lose all your data during the operation so backup your data before replacement.
You will have to take the phone apart, a special screw driver may be required for the screws.
Unscrew all screws and look for screws under soft objects, you probably don't have to unscrew the screws that you find on the expendable keypad, if the unit doesn't open up, then you probably have to take the keypad apart aswel. small screw drivers required.
Here is what the battery looks like, this is from another model from the same maker.
the details should read on top of the battery, I had this phone apart (the one on the picture) few weeks ago and Saw it, I did not take the details from it.
The battery details are not on the spare part list of the service manual either, I would have to open my phone up to get the details about the battery.