I have a 2008 HHR with less than 6K miles and the battery on one of the key fobs is low. This apparently causes the alarm to sound at 3A.M. Can anyone tell me how to replace the battery in the fob? Or do I have to take it to the dealership?

It's quite easy to change a battery, butYou may have to get it re-programmed.An '08 should certainly still be under warranty - can't imagine a Dealership that would notreplace it( Free )for You !

The programming shouldn't be thrown off.

There may be two little screws on the fob, if not, you can just snap it apart with a small screwdriver. There will be a miniscule battery in there. It may be hard to find, but I am fairly certain you can buy them at a walmart or walgreens..

I just take mine into pep boys and they replace battery for me; cost of battery is under $5.

as far as the HHR key fob reset; you have to take into dealer or locksmith that can reset it because it's newer vehicle. The older vehicle key fobs you can set yourself.
I did find someone to do it for $20 versus $30+ at dealership. thanx anyway for help.