The Stable update V12.5.4.0.RJJCNXM has been officially released for Mi 10 Ultra (codename is cas). Before updating, please make sure you have downloaded the exact ROM matching your Xiaomi phone/tablet. In Fastboot mode, you can check model code by typing fastboot getvar product in cmd/shell.
The firmware files posted here are all official Xiaomi MIUI or HyperOS ROM, xiaomiROM.com do not own and never modified those firmwares，you will always download the file from Xiaomi server.
You may interest in those contents: Mi 10 Ultra ROM Topics Xiaomi Devices Index
Download Mi 10 ultra (cas) China MIUI 12.5 V12.5.4.0.RJJCNXM Recovery ROM with Android 11.0. Firmware size is 3.9 GB.