The Xiaomi system has set the bell, but it hasn't changed.

Click on the contact or address book to enter the group interface, click on the group where you want to edit the ringtone, and then you can see who is in this group. Click the function key at the bottom left of the phone in this interface, and a dialog box will pop up with the option of "setting ringtones in batches". The rest is to select the ringtone you want.