Depends on the device. First, if it is the typical tablet it is probably not easily opened, so I would be careful should you try to do this yourself. Now if it is under warranty, contact the manufacturer about it and don't try it yourself. If, however, it isn't under warranty any longer, then you can either search for steps or videos online for your particular tablet on how to change out the battery. Or, should you not feel you can do it yourself, you can try a local tech.
it depends on whether it's possible or not to take out the old battery as most of the devices now has built in battery that cannot be easily out taken. With some tutorial videos it makes it possible to replace the battery with ease.