A UPS is an Uninterruptible Power Supply that provides backup power when the main power source is lost. Questions on repairing commercial ones are generally off topic.
A UPS typically has an AC to DC converter at the input, followed by a battery charger and batteries, and a DC to AC inverter at the output. When the main AC power source is lost power is provided to the attached device(s) via the batteries and inverter. When power is restored the battery is recharged ready for the next power loss event.