Some tablets can be tricky to dismantle. You can try to separate the front from rear case. The battery is usually connected with a connector, rather than soldered in, but there is no guarantee. It is also often glued to either the case back or the board itself, or some other internal structure. You may be able to find a replacement, but they are not really generic, like a C cell, AAA battery or a CR2035 coin cell. Once you get it apart, try to find a number on the battery itself and search for that, or by make/model of the tablet.