Hello!  If its a desktop, you just have to pop open the cover and find the coin cell battery and swap it out, however if you have had your motherboard replaced recently, then it should have a new battery in it.  If its a laptop, technically those have to be replaced by a tech since you have to take the entire bottom off, but then again it should have a new battery in it when the motherboard was replaced.  I would be more conserned that the replacement parts have failed somehow.