Actually, many devices 'say' the battery is not removable, and there is no simple door to access it, however that does not mean it isn't removable. In most cases, it can be. It can just take a lot of effort and a bit of skill to get to it. So unless it is under warranty, or you are worried you could mess something up, then there is no reason to not try.
Also, companies will advertise the battery as 'non-removable' not because you can't do it yourself (in many cases, and with effort and skill) but because they want you to either pay them to have it done (which means they open it up and replace it) or get a new device.