The Weekly update 8.6.21 has been officially released for Mi 6 (codename is sagit). 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 6 ROM Topics Xiaomi Devices Index
Download Mi 6 (sagit) China MIUI 8.6.21 Recovery ROM with Android 8.0. Firmware size is 1.8 GB.
Download Mi 6 (sagit) China MIUI 8.6.21 Fastboot ROM with Android 8.0. Firmware size is 2.8 GB.