help android 7 5.1

Hi, how are you? I'm trying the latest version of MIUI 8.2 (Android 7) and I want to go back to Android 5.1 I'd like to know what method I can use to get back to that Android. I do not know if using the TWRP or MIFlash method via fastboot And one more thing, I use the ROMs of Xiaomi.eu since I...