If your data center was properly designed, you will have a "maintenance bypass" which allows you to route power around the UPS if it's offline for service without interrupting power to the floor.
In most cases, if you have a generator, you may want to run on generator power during the time the UPS is being serviced as the generator power is often cleaner and more reliable than that from the utility.
Start the generator beforehand and allow it to fully warm up. While the UPS is still functional, transfer the load to it and allow it to run that way for 15-30 minutes; if there are any problems, transfer back and cancel the maintenance. While the generator is suppling the load and before shutting down the UPS, check the readings (water, oil pressure, etc.) at the generator engine display and make sure they are all within normal range. And needless to say, check your fuel tank supply level!
If you don't have a generator, watch the weather forecast and conditions closely. If bad weather is called for of present (especially T-storms or high winds), reschedule the maintenance for another day.