Doesn't look like you can (http

/en.miui.com/thread-557253-1-1.html). Best bet is to a) download any photos/videos you don't need day-to-day to your PC or cloud storage (or delete them if you don't want them anymore), b) move any other photos/videos you want to keep for day-to-day use to the SD card, c) move as much of your music as possible to the SD card, & then d) try installing PUBG.