The battery you need should be listed in the manual for the device. If you did not get one, you can check the RCA support site and it should be there.
Also, if the device is under warranty, you should contact the manufacturer about a battery rather than trying to do it yourself, as that would void the warranty.
If it isn't under warranty and you choose to do the work yourself, just be careful of the battery you get to replace the non-working one. Some aftermarket batteries can be hit or miss.
Lastly, you may want to try doing an internet search for videos on how to do the replacement, to walk you through it.