I am using Android so plug your phone in your PC, use File Explorer head to Android/data/com.xiaomi.hm.health/files/watch_skin_local
Jump in to your earlier downloaded theme folder (you can find it by the thumbnail image inside)
Copy that theme folder's name and rename the _packed.bin file with that (ex. 4lIYUjzg3Vwqt8tJU9sYLeOSqUSAreJNzUxJhxgM.bin)
Replace the original file with the modified file
Open Mi Fit and sync the watch face with the band
For advanced users: you can edit the .json files to change position of elements!
That's it. Feel free to ask any questions below. All credit goes to BandBBS.
I hope browser based tools like the one for Amazfit will be available soon, that'll be much easier
p/s: if anyone using iOS please try and comment below since I don't have any iOS devices
It is al the last version, I don't have updates. And the band is also updated to the last version 22.214.171.124.
The only way to update a watchface is to replace the .bin file of a downloaded one (from inside the mifit app) and select it in mifit app after the replacement.
The one downloaded with your app are not displayed there (they also don't have an .xml file inside their folder, but even coping one from the folder of a dowloaded watchface doesn't make it appear)