Comtec is correct. But it's a matter of technical ability and perspective. Replacing the entire unit is the EMC way ( and if you have a contract with them, that is the way to go. However, personally, I wouldn't throw away a ups or pay an exorbitant amount of money when the only problem is a defective battery. We could get ridiculous with this and say that anything with a battery should be replaced when the unit dies... cars, flashlights.. radios..watches..
If you have any doubt at all about your ability to replace the unit, pay EMC or a third party to do it for you. You may even be able to take it to a battery store and ask them to test the batteries. The battery store has the added benefit of disposing of the hazardous materials. My two cents.