No,you can't replace the VRLA batteries with lithium batteries on your existing UPS. Lithium ion batteries have different charging regimes and voltages. If the UPS came with a standard gel type lead acid battery, it will have voltage regulation and charging cycles specific for that type of battery.

There are risk to replace VRLA battries with lithium batteries for your UPS.We would recommend not to switch the battery types. You'd better buy a UPS specially work with lithium batteries. lithium-ion UPS has custom designed BMS (Battery Management System) specifically for lithium-ion batteries, which manages a real-time control of each battery cell and can protect your batteries.