It's best to put a link or photo to the bike in question, I suppose it's this bike? The case will have screws that hold it together. Inside, you'll find a controller ( a silver box with wires) and a battery. Do not know what the battery looks like. The photo shows what it might be,

Just work carefully when you open up the case. Keep the fasteners somewhere where you won't lose them or step on the bottle/box that you put them inside. Take photos of wiring, in case there is any question of what goes where.

At least post a picture here of the battery when you see it, so that there is somewhere on the internet that shows a photo of the darn thing,

As for the battery, It will probably be a rectangular brick with two pairs of wires, similar to the picture. One pair will be heavier duty, and have a large yellow connector. This is the main power line, The connector pulls apart, and you can check the terminals for voltage. The other pair will go to the charger input of the case



The batteries are pretty simple in these bikes. If it were not for product safety considerations, almost any 36V lithium battery of similar size will work. If you have a vendor you trust, easily replaced.