UPS (Uninterruptible Power Supply aka Battery Backup) units use mini lead acid batteries like you would find in your car. Like your car, these batteries only have an expected lifespan of 3-5 years.
Most UPS batteries are fairly easily replaced, though it usually doesn't make sense to have us do it. The cost of labor almost always far exceeds the savings.
If you want to give it a try, check BatterySharks.com and replaceUPSbattery.com to get an idea for the possible cost of replacements batteries