From here.
For some historical reason, my mainly used “China Mobile” phone number is associated to an abondoned XiaoMi account, while my currently using XiaoMi account is associated to a seldom used “China Unicom” phone number. What I want to do is: release my “China Mobile” phone number from the abandoned XiaoMi account, and replace the “China Unicom” phone number with the “China Mobile” one for the XiaoMi account I’m using.
Remove the abondoned XiaoMi account associated to the “China Mobile” phone number entirely. First, login the abondoned XiaoMi account with the “China Mobile” number, backup all the MiCloud data(contacts, photos, audio records and etc, note that a extra tool “Mi Could Photo Manager“ provided by XiaoMi should be use to backup the data in MiCloud, not only photo, but also other types of data. The good part is that the tool works very well, the bad part is that it only supports Windows OS), then go to this URL, follow the given instructions to remove the XiaoMi account entirely.
Login “mi.com” with the “China Unicom” phone number account. Go to “个人中心 -> 个人信息 -> Change recovery phone”, update the phone number from “China Unicom” to “China Mobile”, follow the instructions and you phone number will be updated. After that I can use my “China Mobile” phone number to login the XiaoMi account I’m using.